refactor(response_parsing_test): remove unused error import

Author: SelaseKay

packages/firebase_ai/firebase_ai/test/response_parsing_test.dart

@@ -16,7 +16,6 @@ import 'dart:convert';
 
 import 'package:firebase_ai/firebase_ai.dart';
 import 'package:firebase_ai/src/api.dart';
-import 'package:firebase_ai/src/error.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import 'utils/matchers.dart';