internal/mcp: generic handling of CallToolParams

Make Tool APIs future-proof by pushing generics into CallToolParams, and
using CallToolParams throughout. This has several advantages:

- We no longer need the awkward CallToolOptions type, which is almost
  but not quite like CallToolParams.
- NewTool no longer needs to define its own handler type: it accepts a
  ToolHandler[TArgs]. This also means its signature is future-proof.

Additionally, provide a `CallTool` helper (the analog of NewTool) to
serve as a more strongly typed variant of `ClientSession.CallTool`.

+ design, protocol generation

Change-Id: I80172a25bc50734dae84a541aa4776ed9161694e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676495
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/protocol.go

@@ -7,8 +7,6 @@
 package mcp
 
 import (
-	"encoding/json"
-
 	"github.com/modelcontextprotocol/go-sdk/jsonschema"
 )
 
@@ -28,15 +26,15 @@ type Annotations struct {
 	Priority float64 `json:"priority,omitempty"`
 }
 
-type CallToolParams struct {
+type CallToolParams[TArgs any] struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
-	Meta      Meta            `json:"_meta,omitempty"`
-	Arguments json.RawMessage `json:"arguments,omitempty"`
-	Name      string          `json:"name"`
+	Meta      Meta   `json:"_meta,omitempty"`
+	Arguments TArgs  `json:"arguments,omitempty"`
+	Name      string `json:"name"`
 }
 
-func (x *CallToolParams) GetMeta() *Meta { return &x.Meta }
+func (x *CallToolParams[TArgs]) GetMeta() *Meta { return &x.Meta }
 
 // The server's response to a tool call.
 //