mcp: move to 2025-06-18 schema (#8)

* mcp: move to 2025-06-18 schema

Add types and fields for the latest MCP schema.

The main challenge here was dealing with the presence of _meta fields
on many types, like Tool, Prompt and Resource.

- Distinguish between the _meta field of a request's params, which
  includes a progress token, from the _meta field on other types,
  which is simply a map.

- Redefine GetMeta to return a pointer to the map. We wanted to avoid
  that, but I don't see a clean way to do so. An alternative would be
  to have GetMeta return a map, and SetMeta to set it.

- Provide GetProgressToken for requests only (not notifications or other
  types).

- Still to do: deal with the meta and annotation fields of ContentBlock,
  which are currently duplicated for Resource and ResourceContents.

* Meta and progress token redesign

mcp/shared_test.go

@@ -9,67 +9,8 @@ import (
 	"encoding/json"
 	"strings"
 	"testing"
-
-	"github.com/google/go-cmp/cmp"
 )
 
-func TestMetaMarshal(t *testing.T) {
-	// Verify that Meta values round-trip.
-	for _, meta := range []Meta{
-		{Data: nil, ProgressToken: nil},
-		{Data: nil, ProgressToken: "p"},
-		{Data: map[string]any{"d": true}, ProgressToken: nil},
-		{Data: map[string]any{"d": true}, ProgressToken: "p"},
-	} {
-		got := roundTrip(t, meta)
-		if !cmp.Equal(got, meta) {
-			t.Errorf("\ngot  %#v\nwant %#v", got, meta)
-		}
-	}
-
-	// Check errors.
-	for _, tt := range []struct {
-		meta Meta
-		want string
-	}{
-		{
-			Meta{Data: map[string]any{"progressToken": "p"}, ProgressToken: 1},
-			"duplicate",
-		},
-		{
-			Meta{ProgressToken: true},
-			"bad type",
-		},
-	} {
-		_, err := json.Marshal(tt.meta)
-		if err == nil || !strings.Contains(err.Error(), tt.want) {
-			t.Errorf("%+v: got %v, want error containing %q", tt.meta, err, tt.want)
-		}
-	}
-
-	// Accept progressToken in map if the field is nil.
-	// It will unmarshal by populating ProgressToken.
-	meta := Meta{Data: map[string]any{"progressToken": "p"}}
-	got := roundTrip(t, meta)
-	want := Meta{ProgressToken: "p"}
-	if !cmp.Equal(got, want) {
-		t.Errorf("got %+v, want %+v", got, want)
-	}
-}
-
-func roundTrip[T any](t *testing.T, v T) T {
-	t.Helper()
-	bytes, err := json.Marshal(v)
-	if err != nil {
-		t.Fatal(err)
-	}
-	var res T
-	if err := json.Unmarshal(bytes, &res); err != nil {
-		t.Fatal(err)
-	}
-	return res
-}
-
 // TODO(jba): this shouldn't be in this file, but tool_test.go doesn't have access to unexported symbols.
 func TestNewServerToolValidate(t *testing.T) {
 	// Check that the tool returned from NewServerTool properly validates its input schema.