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/lib/features/chat/data/models/tool_result.dart

@@ -2,6 +2,9 @@ import 'package:json_annotation/json_annotation.dart';
 
 part 'tool_result.g.dart';
 
+/// Schema 版本常量
+const _defaultSchemaVersion = 'v1';
+
 /// 工具执行结果（给 AI 的原始数据）
 @JsonSerializable()
 class ToolResult {
@@ -31,7 +34,7 @@ class UiCard {
 
   UiCard({
     required this.cardType,
-    this.schemaVersion = 'v1',
+    this.schemaVersion = _defaultSchemaVersion,
     required this.data,
     this.actions,
   });