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/content_test.go

@@ -14,7 +14,7 @@ import (
 
 func TestContent(t *testing.T) {
 	tests := []struct {
-		in   *mcp.Content
+		in   *mcp.ContentBlock
 		want string // json serialization
 	}{
 		{mcp.NewTextContent("hello"), `{"type":"text","text":"hello"}`},
@@ -27,13 +27,13 @@ func TestContent(t *testing.T) {
 			`{"type":"audio","mimeType":"audio/wav","data":"YTFiMmMz"}`,
 		},
 		{
-			mcp.NewResourceContent(
+			mcp.NewResourceContents(
 				mcp.NewTextResourceContents("file://foo", "text", "abc"),
 			),
 			`{"type":"resource","resource":{"uri":"file://foo","mimeType":"text","text":"abc"}}`,
 		},
 		{
-			mcp.NewResourceContent(
+			mcp.NewResourceContents(
 				mcp.NewBlobResourceContents("file://foo", "image/png", []byte("a1b2c3")),
 			),
 			`{"type":"resource","resource":{"uri":"file://foo","mimeType":"image/png","blob":"YTFiMmMz"}}`,
@@ -48,7 +48,7 @@ func TestContent(t *testing.T) {
 		if diff := cmp.Diff(test.want, string(got)); diff != "" {
 			t.Errorf("json.Marshal(%v) mismatch (-want +got):\n%s", test.in, diff)
 		}
-		var out *mcp.Content
+		var out *mcp.ContentBlock
 		if err := json.Unmarshal(got, &out); err != nil {
 			t.Fatal(err)
 		}