fix(chat): fix ChatBloc event callback and test reliability

- Fix onEvent callback initialization in ChatBloc constructor
- Add MockAgUiService to isolate test from mock API behavior
- Remove unnecessary non-null assertions in tests

apps/lib/features/chat/presentation/bloc/chat_bloc.dart

@@ -5,6 +5,7 @@ import 'package:flutter_bloc/flutter_bloc.dart';
 import '../../data/models/ag_ui_event.dart';
 import '../../data/models/chat_list_item.dart';
 import '../../data/models/tool_result.dart';
+import '../../data/services/ag_ui_service.dart';
 
 class ChatState {
   final List<ChatListItem> items;
@@ -38,20 +39,12 @@ class ChatState {
   }
 }
 
-class AgUiService {
-  void Function(AgUiEvent)? onEvent;
-
-  AgUiService({this.onEvent});
-
-  Future<void> sendMessage(String content) async {}
-}
-
 class ChatBloc extends Cubit<ChatState> {
   final AgUiService _service;
   final Map<String, String> _toolCallArgsBuffer = {};
 
   ChatBloc({AgUiService? service})
-    : _service = service ?? AgUiService(onEvent: (_) {}),
+    : _service = service ?? AgUiService(),
       super(const ChatState()) {
     _service.onEvent = _handleEvent;
   }