mcp: Implement stateless server (SEP-2275) (#965)

## Description

This PR lays the foundational server-side groundwork for the `>=
2026-06-30` sessionless and stateless feature introduced by
[SEP-2575](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2575)
and SEP-2567, tracked in
[design/stateless.md](614460a225/design/stateless.md).

### SEP-2575: Stateless MCP

*   **Per-request protocol detection in `ServerSession.handle()`**
Unmarshal `_meta` from the raw JSON-RPC params determines whether each
request follows the new protocol.

*   **Per-request typed accessors on `ServerRequest[P]`**
    ```go
    func (r *ServerRequest[P]) ProtocolVersion() string
    func (r *ServerRequest[P]) ClientInfo() *Implementation
    func (r *ServerRequest[P]) ClientCapabilities() *ClientCapabilities
    ```

* **Reject client->server `initialize`, `initialized` and `ping` for
new-protocol requests**

*   **Per-request `_meta` field name constants**
Three constants for the wire-protocol field names
(`MetaKeyProtocolVersion`, `MetaKeyClientInfo`,
`MetaKeyClientCapabilities`)

* **Stop synthesizing fake `InitializeParams` for new-protocol
requests**

Fixes: #966

mcp/protocol.go

@@ -165,10 +165,12 @@ func (x *CallToolResult) UnmarshalJSON(data []byte) error {
 }
 
 func (x *CallToolParams) isParams()              {}
+func (x *CallToolParams) isNil() bool            { return x == nil }
 func (x *CallToolParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *CallToolParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
 func (x *CallToolParamsRaw) isParams()              {}
+func (x *CallToolParamsRaw) isNil() bool            { return x == nil }
 func (x *CallToolParamsRaw) GetProgressToken() any  { return getProgressToken(x) }
 func (x *CallToolParamsRaw) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -187,6 +189,7 @@ type CancelledParams struct {
 }
 
 func (x *CancelledParams) isParams()              {}
+func (x *CancelledParams) isNil() bool            { return x == nil }
 func (x *CancelledParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *CancelledParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -374,7 +377,8 @@ type CompleteParams struct {
 	Ref      *CompleteReference     `json:"ref"`
 }
 
-func (*CompleteParams) isParams() {}
+func (x *CompleteParams) isParams()   {}
+func (x *CompleteParams) isNil() bool { return x == nil }
 
 type CompletionResultDetails struct {
 	HasMore bool     `json:"hasMore,omitempty"`
@@ -422,6 +426,7 @@ type CreateMessageParams struct {
 }
 
 func (x *CreateMessageParams) isParams()              {}
+func (x *CreateMessageParams) isNil() bool            { return x == nil }
 func (x *CreateMessageParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *CreateMessageParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -448,6 +453,7 @@ type CreateMessageWithToolsParams struct {
 }
 
 func (x *CreateMessageWithToolsParams) isParams()              {}
+func (x *CreateMessageWithToolsParams) isNil() bool            { return x == nil }
 func (x *CreateMessageWithToolsParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *CreateMessageWithToolsParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -654,6 +660,7 @@ type GetPromptParams struct {
 }
 
 func (x *GetPromptParams) isParams()              {}
+func (x *GetPromptParams) isNil() bool            { return x == nil }
 func (x *GetPromptParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *GetPromptParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -706,6 +713,7 @@ func (p *initializeParamsV2) toV1() *InitializeParams {
 }
 
 func (x *InitializeParams) isParams()              {}
+func (x *InitializeParams) isNil() bool            { return x == nil }
 func (x *InitializeParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *InitializeParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -739,6 +747,7 @@ type InitializedParams struct {
 }
 
 func (x *InitializedParams) isParams()              {}
+func (x *InitializedParams) isNil() bool            { return x == nil }
 func (x *InitializedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *InitializedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -752,6 +761,7 @@ type ListPromptsParams struct {
 }
 
 func (x *ListPromptsParams) isParams()              {}
+func (x *ListPromptsParams) isNil() bool            { return x == nil }
 func (x *ListPromptsParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ListPromptsParams) SetProgressToken(t any) { setProgressToken(x, t) }
 func (x *ListPromptsParams) cursorPtr() *string     { return &x.Cursor }
@@ -780,6 +790,7 @@ type ListResourceTemplatesParams struct {
 }
 
 func (x *ListResourceTemplatesParams) isParams()              {}
+func (x *ListResourceTemplatesParams) isNil() bool            { return x == nil }
 func (x *ListResourceTemplatesParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ListResourceTemplatesParams) SetProgressToken(t any) { setProgressToken(x, t) }
 func (x *ListResourceTemplatesParams) cursorPtr() *string     { return &x.Cursor }
@@ -808,6 +819,7 @@ type ListResourcesParams struct {
 }
 
 func (x *ListResourcesParams) isParams()              {}
+func (x *ListResourcesParams) isNil() bool            { return x == nil }
 func (x *ListResourcesParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ListResourcesParams) SetProgressToken(t any) { setProgressToken(x, t) }
 func (x *ListResourcesParams) cursorPtr() *string     { return &x.Cursor }
@@ -833,6 +845,7 @@ type ListRootsParams struct {
 }
 
 func (x *ListRootsParams) isParams()              {}
+func (x *ListRootsParams) isNil() bool            { return x == nil }
 func (x *ListRootsParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ListRootsParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -858,6 +871,7 @@ type ListToolsParams struct {
 }
 
 func (x *ListToolsParams) isParams()              {}
+func (x *ListToolsParams) isNil() bool            { return x == nil }
 func (x *ListToolsParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ListToolsParams) SetProgressToken(t any) { setProgressToken(x, t) }
 func (x *ListToolsParams) cursorPtr() *string     { return &x.Cursor }
@@ -896,6 +910,7 @@ type LoggingMessageParams struct {
 }
 
 func (x *LoggingMessageParams) isParams()              {}
+func (x *LoggingMessageParams) isNil() bool            { return x == nil }
 func (x *LoggingMessageParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *LoggingMessageParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -958,6 +973,7 @@ type PingParams struct {
 }
 
 func (x *PingParams) isParams()              {}
+func (x *PingParams) isNil() bool            { return x == nil }
 func (x *PingParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *PingParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -978,7 +994,8 @@ type ProgressNotificationParams struct {
 	Total float64 `json:"total,omitempty"`
 }
 
-func (*ProgressNotificationParams) isParams() {}
+func (x *ProgressNotificationParams) isParams()   {}
+func (x *ProgressNotificationParams) isNil() bool { return x == nil }
 
 // IconTheme specifies the theme an icon is designed for.
 type IconTheme string
@@ -1048,6 +1065,7 @@ type PromptListChangedParams struct {
 }
 
 func (x *PromptListChangedParams) isParams()              {}
+func (x *PromptListChangedParams) isNil() bool            { return x == nil }
 func (x *PromptListChangedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *PromptListChangedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -1089,6 +1107,7 @@ type ReadResourceParams struct {
 }
 
 func (x *ReadResourceParams) isParams()              {}
+func (x *ReadResourceParams) isNil() bool            { return x == nil }
 func (x *ReadResourceParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ReadResourceParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -1145,6 +1164,7 @@ type ResourceListChangedParams struct {
 }
 
 func (x *ResourceListChangedParams) isParams()              {}
+func (x *ResourceListChangedParams) isNil() bool            { return x == nil }
 func (x *ResourceListChangedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ResourceListChangedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -1205,6 +1225,7 @@ type RootsListChangedParams struct {
 }
 
 func (x *RootsListChangedParams) isParams()              {}
+func (x *RootsListChangedParams) isNil() bool            { return x == nil }
 func (x *RootsListChangedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *RootsListChangedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -1288,6 +1309,7 @@ type SetLoggingLevelParams struct {
 }
 
 func (x *SetLoggingLevelParams) isParams()              {}
+func (x *SetLoggingLevelParams) isNil() bool            { return x == nil }
 func (x *SetLoggingLevelParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *SetLoggingLevelParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -1416,6 +1438,7 @@ type ToolListChangedParams struct {
 }
 
 func (x *ToolListChangedParams) isParams()              {}
+func (x *ToolListChangedParams) isNil() bool            { return x == nil }
 func (x *ToolListChangedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ToolListChangedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -1429,7 +1452,8 @@ type SubscribeParams struct {
 	URI string `json:"uri"`
 }
 
-func (*SubscribeParams) isParams() {}
+func (x *SubscribeParams) isParams()   {}
+func (x *SubscribeParams) isNil() bool { return x == nil }
 
 // Sent from the client to request cancellation of resources/updated
 // notifications from the server. This should follow a previous
@@ -1442,7 +1466,8 @@ type UnsubscribeParams struct {
 	URI string `json:"uri"`
 }
 
-func (*UnsubscribeParams) isParams() {}
+func (x *UnsubscribeParams) isParams()   {}
+func (x *UnsubscribeParams) isNil() bool { return x == nil }
 
 // A notification from the server to the client, informing it that a resource
 // has changed and may need to be read again. This should only be sent if the
@@ -1455,7 +1480,8 @@ type ResourceUpdatedNotificationParams struct {
 	URI string `json:"uri"`
 }
 
-func (*ResourceUpdatedNotificationParams) isParams() {}
+func (x *ResourceUpdatedNotificationParams) isParams()   {}
+func (x *ResourceUpdatedNotificationParams) isNil() bool { return x == nil }
 
 // TODO(jba): add CompleteRequest and related types.
 
@@ -1494,7 +1520,8 @@ type ElicitParams struct {
 	ElicitationID string `json:"elicitationId,omitempty"`
 }
 
-func (x *ElicitParams) isParams() {}
+func (x *ElicitParams) isParams()   {}
+func (x *ElicitParams) isNil() bool { return x == nil }
 
 func (x *ElicitParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ElicitParams) SetProgressToken(t any) { setProgressToken(x, t) }
@@ -1526,7 +1553,8 @@ type ElicitationCompleteParams struct {
 	ElicitationID string `json:"elicitationId"`
 }
 
-func (*ElicitationCompleteParams) isParams() {}
+func (x *ElicitationCompleteParams) isParams()   {}
+func (x *ElicitationCompleteParams) isNil() bool { return x == nil }
 
 // An Implementation describes the name and version of an MCP implementation, with an optional
 // title for UI representation.
@@ -1656,3 +1684,17 @@ const (
 	notificationToolListChanged     = "notifications/tools/list_changed"
 	methodUnsubscribe               = "resources/unsubscribe"
 )
+
+// Per-request _meta field names for the >= 2026-06-30 protocol version.
+//
+// These keys appear inside a Params._meta map and carry information that
+// previously came from the initialization handshake (SEP-2575).
+const (
+	// MetaKeyProtocolVersion identifies the MCP protocol version that the
+	// request follows.
+	MetaKeyProtocolVersion = "io.modelcontextprotocol/protocolVersion"
+	// MetaKeyClientInfo carries the client's [Implementation].
+	MetaKeyClientInfo = "io.modelcontextprotocol/clientInfo"
+	// MetaKeyClientCapabilities carries the client's [ClientCapabilities].
+	MetaKeyClientCapabilities = "io.modelcontextprotocol/clientCapabilities"
+)