mcp: add a streamable serving benchmark

Add a benchmark using the streamable HTTP handler, and a more realistic input/output signature. Also expose this handler as an example (both a go example and a server in the examples/ directory).

mcp/event_test.go

@@ -287,7 +287,7 @@ func BenchmarkMemoryEventStore(b *testing.B) {
 			payload := make([]byte, test.datasize)
 			start := time.Now()
 			b.ResetTimer()
-			for i := 0; i < b.N; i++ {
+			for i := range b.N {
 				sessionID := sessionIDs[i%len(sessionIDs)]
 				streamID := streamIDs[i%len(sessionIDs)][i%3]
 				store.Append(ctx, sessionID, streamID, payload)