mcp: include resultType on new-protocol responses (#1060)

## Summary

- Add `resultType: "complete"` to non-MRTR server result responses for
2026-07-28 requests.
- Cover `server/discover`, list methods, and completion responses, and
update expected conformance/example output.

## Why

Fixes #1032. The 2026-07-28 schema requires `resultType` on result
objects, but only the MRTR-capable results were setting it.

## Validation

- `go test ./mcp -run
'Test(ServerSessionHandle|CompleteResult|ClientConnectDiscover|InMemory_E2E_Discover)'
-count=1` — passed\n- `go test ./mcp -count=1` — passed\n- `go test
./...` — passed\n- `go vet ./...` — passed

---------

Co-authored-by: ychampion <ychampion@users.noreply.github.com>
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

mcp/protocol.go

@@ -28,6 +28,24 @@ const (
 	resultTypeInputRequired resultType = "input_required"
 )
 
+type completeResultWithType struct {
+	ResultType resultType `json:"resultType,omitempty"`
+}
+
+func (r *completeResultWithType) setResultType(rt resultType) { r.ResultType = rt }
+func (*completeResultWithType) isCompleteResult()             {}
+
+type completeResultResponse interface {
+	setResultType(resultType)
+	isCompleteResult()
+}
+
+func setCompleteResultType(res Result) {
+	if r, ok := res.(completeResultResponse); ok {
+		r.setResultType(resultTypeComplete)
+	}
+}
+
 // InputRequest is a type for parameters that a server can include in the response
 // to request input from client (SEP-2322). Implementations are [*ElicitParams],
 // [*CreateMessageParams], and [*ListRootsParams].
@@ -640,6 +658,7 @@ type CompletionResultDetails struct {
 
 // The server's response to a completion/complete request
 type CompleteResult struct {
+	completeResultWithType
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta       `json:"_meta,omitempty"`
@@ -1117,6 +1136,7 @@ func (x *DiscoverParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *DiscoverParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
 type DiscoverResult struct {
+	completeResultWithType
 	Meta `json:"_meta,omitempty"`
 	Cacheable
 	// The versions of the Model Context Protocol that the server supports.
@@ -1180,6 +1200,7 @@ func (c *Cacheable) setDefaultCacheableValues() {
 
 // The server's response to a prompts/list request from the client.
 type ListPromptsResult struct {
+	completeResultWithType
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta `json:"_meta,omitempty"`
@@ -1210,6 +1231,7 @@ func (x *ListResourceTemplatesParams) cursorPtr() *string     { return &x.Cursor
 
 // The server's response to a resources/templates/list request from the client.
 type ListResourceTemplatesResult struct {
+	completeResultWithType
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta `json:"_meta,omitempty"`
@@ -1240,6 +1262,7 @@ func (x *ListResourcesParams) cursorPtr() *string     { return &x.Cursor }
 
 // The server's response to a resources/list request from the client.
 type ListResourcesResult struct {
+	completeResultWithType
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta `json:"_meta,omitempty"`
@@ -1306,6 +1329,7 @@ func (x *ListToolsParams) cursorPtr() *string     { return &x.Cursor }
 
 // The server's response to a tools/list request from the client.
 type ListToolsResult struct {
+	completeResultWithType
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta `json:"_meta,omitempty"`