fix: change deprecated func (#4236)

apps/backend-mock/error.ts

@@ -1,7 +1,7 @@
 import type { NitroErrorHandler } from 'nitropack';
 
 const errorHandler: NitroErrorHandler = function (error, event) {
-  event.res.end(`[error handler] ${error.stack}`);
+  event.node.res.end(`[Error Handler] ${error.stack}`);
 };
 
 export default errorHandler;