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.dart

@@ -9,6 +9,8 @@ class AgUiEventTypeWire {
   static const runStarted = 'RUN_STARTED';
   static const runFinished = 'RUN_FINISHED';
   static const runError = 'RUN_ERROR';
+  static const stepStarted = 'STEP_STARTED';
+  static const stepFinished = 'STEP_FINISHED';
   static const textMessageStart = 'TEXT_MESSAGE_START';
   static const textMessageContent = 'TEXT_MESSAGE_CONTENT';
   static const textMessageEnd = 'TEXT_MESSAGE_END';
@@ -25,6 +27,8 @@ enum AgUiEventType {
   runStarted,
   runFinished,
   runError,
+  stepStarted,
+  stepFinished,
   textMessageStart,
   textMessageContent,
   textMessageEnd,
@@ -43,6 +47,8 @@ const _wireToTypeMap = {
   AgUiEventTypeWire.runStarted: AgUiEventType.runStarted,
   AgUiEventTypeWire.runFinished: AgUiEventType.runFinished,
   AgUiEventTypeWire.runError: AgUiEventType.runError,
+  AgUiEventTypeWire.stepStarted: AgUiEventType.stepStarted,
+  AgUiEventTypeWire.stepFinished: AgUiEventType.stepFinished,
   AgUiEventTypeWire.textMessageStart: AgUiEventType.textMessageStart,
   AgUiEventTypeWire.textMessageContent: AgUiEventType.textMessageContent,
   AgUiEventTypeWire.textMessageEnd: AgUiEventType.textMessageEnd,
@@ -60,6 +66,8 @@ const _typeToWireMap = {
   AgUiEventType.runStarted: AgUiEventTypeWire.runStarted,
   AgUiEventType.runFinished: AgUiEventTypeWire.runFinished,
   AgUiEventType.runError: AgUiEventTypeWire.runError,
+  AgUiEventType.stepStarted: AgUiEventTypeWire.stepStarted,
+  AgUiEventType.stepFinished: AgUiEventTypeWire.stepFinished,
   AgUiEventType.textMessageStart: AgUiEventTypeWire.textMessageStart,
   AgUiEventType.textMessageContent: AgUiEventTypeWire.textMessageContent,
   AgUiEventType.textMessageEnd: AgUiEventTypeWire.textMessageEnd,
@@ -83,6 +91,8 @@ final _typeToFactory = {
   AgUiEventType.runStarted: RunStartedEvent.fromJson,
   AgUiEventType.runFinished: RunFinishedEvent.fromJson,
   AgUiEventType.runError: RunErrorEvent.fromJson,
+  AgUiEventType.stepStarted: StepStartedEvent.fromJson,
+  AgUiEventType.stepFinished: StepFinishedEvent.fromJson,
   AgUiEventType.textMessageStart: TextMessageStartEvent.fromJson,
   AgUiEventType.textMessageContent: TextMessageContentEvent.fromJson,
   AgUiEventType.textMessageEnd: TextMessageEndEvent.fromJson,
@@ -170,6 +180,34 @@ class RunErrorEvent extends AgUiEvent {
   Map<String, dynamic> toJson() => _$RunErrorEventToJson(this);
 }
 
+@JsonSerializable()
+class StepStartedEvent extends AgUiEvent {
+  final String stepName;
+
+  StepStartedEvent({required this.stepName})
+    : super(type: AgUiEventType.stepStarted);
+
+  factory StepStartedEvent.fromJson(Map<String, dynamic> json) =>
+      _$StepStartedEventFromJson(json);
+
+  @override
+  Map<String, dynamic> toJson() => _$StepStartedEventToJson(this);
+}
+
+@JsonSerializable()
+class StepFinishedEvent extends AgUiEvent {
+  final String stepName;
+
+  StepFinishedEvent({required this.stepName})
+    : super(type: AgUiEventType.stepFinished);
+
+  factory StepFinishedEvent.fromJson(Map<String, dynamic> json) =>
+      _$StepFinishedEventFromJson(json);
+
+  @override
+  Map<String, dynamic> toJson() => _$StepFinishedEventToJson(this);
+}
+
 @JsonSerializable()
 class TextMessageStartEvent extends AgUiEvent {
   final String messageId;
@@ -310,10 +348,33 @@ class ToolCallResultEvent extends AgUiEvent {
 
   factory ToolCallResultEvent.fromJson(Map<String, dynamic> json) {
     final rawContent = json['content'];
-    final content = rawContent is String ? rawContent : '';
+    final hasStructuredFields =
+        json['ui'] != null || json['result'] != null || json['error'] != null;
+    final content = switch (rawContent) {
+      String value when value.trim().startsWith('{') => value,
+      String value when value.trim().startsWith('[') => value,
+      String value when hasStructuredFields => jsonEncode({
+        'toolName': json['toolName'],
+        'result': json['result'],
+        'error': json['error'],
+        'ui': json['ui'],
+        'content': value,
+      }),
+      String value => value,
+      _ => jsonEncode({
+        'toolName': json['toolName'],
+        'result': json['result'],
+        'error': json['error'],
+        'ui': json['ui'],
+        'content': json['content'],
+      }),
+    };
+    final toolCallId =
+        json['toolCallId'] as String? ?? json['callId'] as String? ?? '';
+    final messageId = json['messageId'] as String? ?? 'tool-result-$toolCallId';
     return ToolCallResultEvent(
-      messageId: json['messageId'] as String,
-      toolCallId: json['toolCallId'] as String,
+      messageId: messageId,
+      toolCallId: toolCallId,
       content: content,
     );
   }
@@ -388,6 +449,7 @@ class SnapshotMessage {
   final String? toolCallId;
   final UiCard? ui;
   final DateTime? timestamp;
+  final List<Map<String, dynamic>>? attachments;
 
   SnapshotMessage({
     required this.id,
@@ -396,6 +458,7 @@ class SnapshotMessage {
     this.toolCallId,
     this.ui,
     this.timestamp,
+    this.attachments,
   });
 
   factory SnapshotMessage.fromJson(Map<String, dynamic> json) =>