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/mcp_test.go

@@ -34,7 +34,7 @@ func sayHi(ctx context.Context, ss *ServerSession, params *CallToolParamsFor[hiP
 	if err := ss.Ping(ctx, nil); err != nil {
 		return nil, fmt.Errorf("ping failed: %v", err)
 	}
-	return &CallToolResultFor[any]{Content: []*Content{NewTextContent("hi " + params.Arguments.Name)}}, nil
+	return &CallToolResultFor[any]{Content: []*ContentBlock{NewTextContent("hi " + params.Arguments.Name)}}, nil
 }
 
 func TestEndToEnd(t *testing.T) {
@@ -195,7 +195,7 @@ func TestEndToEnd(t *testing.T) {
 			t.Fatal(err)
 		}
 		wantHi := &CallToolResult{
-			Content: []*Content{{Type: "text", Text: "hi user"}},
+			Content: []*ContentBlock{{Type: "text", Text: "hi user"}},
 		}
 		if diff := cmp.Diff(wantHi, gotHi); diff != "" {
 			t.Errorf("tools/call 'greet' mismatch (-want +got):\n%s", diff)
@@ -212,7 +212,7 @@ func TestEndToEnd(t *testing.T) {
 		}
 		wantFail := &CallToolResult{
 			IsError: true,
-			Content: []*Content{{Type: "text", Text: errTestFailure.Error()}},
+			Content: []*ContentBlock{{Type: "text", Text: errTestFailure.Error()}},
 		}
 		if diff := cmp.Diff(wantFail, gotFail); diff != "" {
 			t.Errorf("tools/call 'fail' mismatch (-want +got):\n%s", diff)