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/lib/core/api/api_client.dart

@@ -50,7 +50,6 @@ class ApiClient implements IApiClient {
     };
   }
 
-  @override
   Future<Response<T>> get<T>(String path, {Options? options}) async {
     try {
       return await _dio.get<T>(path, options: options);

apps/lib/core/api/mock_api_client.dart

@@ -40,7 +40,11 @@ class MockApiClient implements IApiClient {
     _handlers[key] = handler;
   }
 
-  void registerPatternHandler(RegExp pattern, String method, MockHandler handler) {
+  void registerPatternHandler(
+    RegExp pattern,
+    String method,
+    MockHandler handler,
+  ) {
     _patternHandlers.add(
       _PatternRoute(
         pattern: pattern,
@@ -96,11 +100,7 @@ class MockApiClient implements IApiClient {
     final direct = _handlers[key];
     if (direct != null) {
       final response = direct(
-        MockRequest(
-          path: path,
-          method: 'SSE',
-          headers: headers,
-        ),
+        MockRequest(path: path, method: 'SSE', headers: headers),
       );
       if (response is Stream<String>) {
         return response;
@@ -118,11 +118,7 @@ class MockApiClient implements IApiClient {
         continue;
       }
       final response = route.handler(
-        MockRequest(
-          path: path,
-          method: 'SSE',
-          headers: headers,
-        ),
+        MockRequest(path: path, method: 'SSE', headers: headers),
       );
       if (response is Stream<String>) {
         return response;
@@ -147,12 +143,7 @@ class MockApiClient implements IApiClient {
 
     if (handler != null) {
       final response = handler(
-        MockRequest(
-          path: path,
-          method: method,
-          data: data,
-          options: options,
-        ),
+        MockRequest(path: path, method: method, data: data, options: options),
       );
       if (response is Response) {
         return response as Response<T>;