feat: 添加 Agent 步骤事件与图片附件功能

- 新增 stepStarted/stepFinished 事件类型支持
- 前端实现图片附件上传和预览功能
- 后端增强工具结果存储和事件处理
- 完善相关单元测试和集成测试

apps/test/features/home/ui/screens/home_screen_test.dart

@@ -6,11 +6,14 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:image_picker/image_picker.dart';
 import 'package:lucide_icons/lucide_icons.dart';
 import 'package:social_app/core/api/api_exception.dart';
+import 'package:social_app/core/api/mock_api_client.dart';
+import 'package:social_app/core/di/injection.dart';
 import 'package:social_app/features/chat/data/models/ag_ui_event.dart';
 import 'package:social_app/features/chat/data/services/ag_ui_service.dart';
 import 'package:social_app/features/chat/presentation/bloc/chat_bloc.dart';
 import 'package:social_app/features/home/data/voice_recorder.dart';
 import 'package:social_app/features/home/ui/screens/home_screen.dart';
+import 'package:social_app/features/messages/data/inbox_api.dart';
 
 class _FakeVoiceRecorder implements VoiceRecorder {
   bool started = false;
@@ -43,9 +46,19 @@ class _WaitingAgUiService extends AgUiService {
     onEvent(RunStartedEvent(threadId: 't1', runId: 'r1'));
     return _pending.future;
   }
+
+  void emitStepStarted(String stepName) {
+    onEvent(StepStartedEvent(stepName: stepName));
+  }
 }
 
 void main() {
+  setUpAll(() {
+    if (!sl.isRegistered<InboxApi>()) {
+      sl.registerSingleton<InboxApi>(InboxApi(MockApiClient()));
+    }
+  });
+
   IconData _inputActionIcon(WidgetTester tester) {
     final icon = tester.widget<Icon>(
       find.byKey(const ValueKey('home_input_action_icon')),
@@ -275,7 +288,8 @@ void main() {
     testWidgets('shows stop icon and waiting indicator while waiting agent', (
       WidgetTester tester,
     ) async {
-      final chatBloc = ChatBloc(service: _WaitingAgUiService());
+      final waitingService = _WaitingAgUiService();
+      final chatBloc = ChatBloc(service: waitingService);
       await tester.pumpWidget(
         MaterialApp(
           home: HomeScreen(autoLoadHistory: false, chatBloc: chatBloc),
@@ -291,6 +305,12 @@ void main() {
       expect(_inputActionIcon(tester), LucideIcons.square);
       expect(find.text('正在思考...'), findsOneWidget);
 
+      waitingService.emitStepStarted('intent');
+      await tester.pump();
+
+      expect(find.text('意图识别中'), findsOneWidget);
+      expect(find.text('正在思考...'), findsNothing);
+
       await tester.tap(find.byKey(const ValueKey('home_input_action_button')));
       await tester.pump();