fix(mcp): do not respond to JSON-RPC notifications

The supervisor's MCP HTTP handler was answering every parsed request,
including notifications (messages with no id field). Per JSON-RPC 2.0,
notifications must not receive a response. The Apr-29 incident saw
Claude Code's MCP client receive a -32601 error for the standard
notifications/initialized handshake step and disconnect immediately
after a successful initialize.

Skip writing the response when req.ID == nil. Cover both the
known-method (notifications/initialized) and unknown-method paths
with tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/mcp/server.go

@@ -43,6 +43,11 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// JSON-RPC 2.0 notifications (no id) must not receive a response.
+	if req.ID == nil {
+		return
+	}
+
 	var result any
 	var rpcErr *rpcError