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/features/chat/data/models/ag_ui_event.g.dart

@@ -14,6 +14,8 @@ const _$AgUiEventTypeEnumMap = {
   AgUiEventType.runStarted: 'runStarted',
   AgUiEventType.runFinished: 'runFinished',
   AgUiEventType.runError: 'runError',
+  AgUiEventType.stepStarted: 'stepStarted',
+  AgUiEventType.stepFinished: 'stepFinished',
   AgUiEventType.textMessageStart: 'textMessageStart',
   AgUiEventType.textMessageContent: 'textMessageContent',
   AgUiEventType.textMessageEnd: 'textMessageEnd',
@@ -53,6 +55,18 @@ RunErrorEvent _$RunErrorEventFromJson(Map<String, dynamic> json) =>
 Map<String, dynamic> _$RunErrorEventToJson(RunErrorEvent instance) =>
     <String, dynamic>{'message': instance.message, 'code': instance.code};
 
+StepStartedEvent _$StepStartedEventFromJson(Map<String, dynamic> json) =>
+    StepStartedEvent(stepName: json['stepName'] as String);
+
+Map<String, dynamic> _$StepStartedEventToJson(StepStartedEvent instance) =>
+    <String, dynamic>{'stepName': instance.stepName};
+
+StepFinishedEvent _$StepFinishedEventFromJson(Map<String, dynamic> json) =>
+    StepFinishedEvent(stepName: json['stepName'] as String);
+
+Map<String, dynamic> _$StepFinishedEventToJson(StepFinishedEvent instance) =>
+    <String, dynamic>{'stepName': instance.stepName};
+
 TextMessageStartEvent _$TextMessageStartEventFromJson(
   Map<String, dynamic> json,
 ) => TextMessageStartEvent(
@@ -170,6 +184,9 @@ SnapshotMessage _$SnapshotMessageFromJson(Map<String, dynamic> json) =>
       timestamp: json['timestamp'] == null
           ? null
           : DateTime.parse(json['timestamp'] as String),
+      attachments: (json['attachments'] as List<dynamic>?)
+          ?.whereType<Map<String, dynamic>>()
+          .toList(),
     );
 
 Map<String, dynamic> _$SnapshotMessageToJson(SnapshotMessage instance) =>
@@ -180,4 +197,5 @@ Map<String, dynamic> _$SnapshotMessageToJson(SnapshotMessage instance) =>
       'toolCallId': instance.toolCallId,
       'ui': instance.ui,
       'timestamp': instance.timestamp?.toIso8601String(),
+      'attachments': instance.attachments,
     };