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/services/ag_ui_service.dart

@@ -8,6 +8,18 @@ import '../models/ag_ui_event.dart';
 import '../models/tool_result.dart';
 import '../tools/tool_registry.dart';
 
+/// Mock ID 前缀常量
+const _threadIdPrefix = 'thread_';
+const _runIdPrefix = 'run_';
+const _toolCallIdPrefix = 'tc_';
+const _messageIdPrefix = 'msg_';
+
+/// 流式输出延迟 (毫秒)
+const _streamChunkDelayMs = 50;
+
+/// 文本块大小
+const _textChunkSize = 10;
+
 typedef EventCallback = void Function(AgUiEvent event);
 
 class AgUiService {
@@ -27,8 +39,8 @@ class AgUiService {
   }
 
   Future<void> _mockEventStream(String content) async {
-    final threadId = 'thread_${DateTime.now().millisecondsSinceEpoch}';
-    final runId = 'run_${DateTime.now().millisecondsSinceEpoch}';
+    final threadId = '$_threadIdPrefix${DateTime.now().millisecondsSinceEpoch}';
+    final runId = '$_runIdPrefix${DateTime.now().millisecondsSinceEpoch}';
 
     onEvent(RunStartedEvent(threadId: threadId, runId: runId));
 
@@ -58,7 +70,8 @@ class AgUiService {
     String toolName,
     Map<String, dynamic> args,
   ) async {
-    final toolCallId = 'tc_${DateTime.now().millisecondsSinceEpoch}';
+    final toolCallId =
+        '$_toolCallIdPrefix${DateTime.now().millisecondsSinceEpoch}';
 
     onEvent(ToolCallStartEvent(toolCallId: toolCallId, toolCallName: toolName));
 
@@ -82,7 +95,8 @@ class AgUiService {
       ToolRegistry.initialize();
       final result = await ToolRegistry.execute(toolName, args);
       final ui = _buildUiCard(toolName, result);
-      final messageId = 'msg_${DateTime.now().millisecondsSinceEpoch}';
+      final messageId =
+          '$_messageIdPrefix${DateTime.now().millisecondsSinceEpoch}';
 
       onEvent(
         ToolCallResultEvent(
@@ -145,20 +159,20 @@ class AgUiService {
 
   Future<void> _mockTextMessageStream(List<String> replies) async {
     for (final reply in replies) {
-      final messageId = 'msg_${DateTime.now().millisecondsSinceEpoch}';
+      final messageId =
+          '$_messageIdPrefix${DateTime.now().millisecondsSinceEpoch}';
 
       onEvent(TextMessageStartEvent(messageId: messageId, role: 'assistant'));
 
-      const chunkSize = 10;
-      for (var i = 0; i < reply.length; i += chunkSize) {
-        final end = (i + chunkSize < reply.length)
-            ? i + chunkSize
+      for (var i = 0; i < reply.length; i += _textChunkSize) {
+        final end = (i + _textChunkSize < reply.length)
+            ? i + _textChunkSize
             : reply.length;
         final chunk = reply.substring(i, end);
 
         onEvent(TextMessageContentEvent(messageId: messageId, delta: chunk));
 
-        await Future.delayed(const Duration(milliseconds: 50));
+        await Future.delayed(const Duration(milliseconds: _streamChunkDelayMs));
       }
 
       onEvent(TextMessageEndEvent(messageId: messageId));