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/test/features/chat/chat_bloc_test.dart

@@ -2,14 +2,22 @@ import 'package:bloc_test/bloc_test.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:social_app/features/chat/data/models/ag_ui_event.dart';
 import 'package:social_app/features/chat/data/models/chat_list_item.dart';
+import 'package:social_app/features/chat/data/services/ag_ui_service.dart';
 import 'package:social_app/features/chat/presentation/bloc/chat_bloc.dart';
 
+class MockAgUiService extends AgUiService {
+  MockAgUiService() : super(onEvent: (_) {});
+
+  @override
+  Future<void> sendMessage(String content) async {}
+}
+
 void main() {
   late ChatBloc chatBloc;
   late AgUiService service;
 
   setUp(() {
-    service = AgUiService();
+    service = MockAgUiService();
     chatBloc = ChatBloc(service: service);
   });
 
@@ -49,7 +57,7 @@ void main() {
       build: () => chatBloc,
       act: (bloc) {
         bloc.emit(chatBloc.state.copyWith(isLoading: true));
-        service.onEvent!(
+        service.onEvent(
           TextMessageStartEvent(messageId: 'msg_1', role: 'assistant'),
         );
       },
@@ -86,7 +94,7 @@ void main() {
         currentMessageId: 'msg_1',
       ),
       act: (bloc) {
-        service.onEvent!(
+        service.onEvent(
           TextMessageContentEvent(messageId: 'msg_1', delta: 'Hello'),
         );
       },
@@ -115,7 +123,7 @@ void main() {
         currentMessageId: 'msg_1',
       ),
       act: (bloc) {
-        service.onEvent!(TextMessageEndEvent(messageId: 'msg_1'));
+        service.onEvent(TextMessageEndEvent(messageId: 'msg_1'));
       },
       expect: () => [
         isA<ChatState>()
@@ -132,7 +140,7 @@ void main() {
       'runStarted sets isLoading to true',
       build: () => chatBloc,
       act: (bloc) {
-        service.onEvent!(RunStartedEvent(threadId: 't1', runId: 'r1'));
+        service.onEvent(RunStartedEvent(threadId: 't1', runId: 'r1'));
       },
       expect: () => [
         isA<ChatState>()
@@ -146,7 +154,7 @@ void main() {
       build: () => chatBloc,
       seed: () => const ChatState(isLoading: true),
       act: (bloc) {
-        service.onEvent!(RunFinishedEvent(threadId: 't1', runId: 'r1'));
+        service.onEvent(RunFinishedEvent(threadId: 't1', runId: 'r1'));
       },
       expect: () => [
         isA<ChatState>()
@@ -160,7 +168,7 @@ void main() {
       build: () => chatBloc,
       seed: () => const ChatState(isLoading: true),
       act: (bloc) {
-        service.onEvent!(
+        service.onEvent(
           RunErrorEvent(message: 'Something went wrong', code: 'ERR'),
         );
       },
@@ -183,7 +191,7 @@ void main() {
       'toolCallStart adds ToolCallItem',
       build: () => chatBloc,
       act: (bloc) {
-        service.onEvent!(
+        service.onEvent(
           ToolCallStartEvent(
             toolCallId: 'tc_1',
             toolCallName: 'create_calendar_event',