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/tools/tool_registry.dart

@@ -1,6 +1,14 @@
 typedef ToolHandler =
     Future<Map<String, dynamic>> Function(Map<String, dynamic> args);
 
+/// 工具常量
+const _toolNameCreateCalendar = 'create_calendar_event';
+const _defaultTimezone = 'Asia/Shanghai';
+const _defaultEventColor = '#4F46E5';
+const _defaultSourceType = 'agentGenerated';
+const _titleMinLength = 1;
+const _titleMaxLength = 100;
+
 class ToolDefinition {
   final String name;
   final String description;
@@ -22,8 +30,8 @@ class ToolRegistry {
   static void initialize() {
     if (_initialized) return;
 
-    _tools['create_calendar_event'] = ToolDefinition(
-      name: 'create_calendar_event',
+    _tools[_toolNameCreateCalendar] = ToolDefinition(
+      name: _toolNameCreateCalendar,
       description: '创建一个日历事件或待办事项',
       parameters: {
         'type': 'object',
@@ -31,8 +39,8 @@ class ToolRegistry {
           'title': {
             'type': 'string',
             'description': '事件标题',
-            'minLength': 1,
-            'maxLength': 100,
+            'minLength': _titleMinLength,
+            'maxLength': _titleMaxLength,
           },
           'description': {'type': 'string', 'description': '事件描述'},
           'startAt': {
@@ -45,7 +53,7 @@ class ToolRegistry {
             'format': 'date-time',
             'description': '结束时间 (ISO8601)',
           },
-          'timezone': {'type': 'string', 'default': 'Asia/Shanghai'},
+          'timezone': {'type': 'string', 'default': _defaultTimezone},
           'location': {'type': 'string'},
           'notes': {'type': 'string'},
         },
@@ -69,10 +77,10 @@ class ToolRegistry {
       'description': args['description'],
       'startAt': args['startAt'],
       'endAt': args['endAt'],
-      'timezone': args['timezone'] ?? 'Asia/Shanghai',
+      'timezone': args['timezone'] ?? _defaultTimezone,
       'location': args['location'],
-      'color': '#4F46E5',
-      'sourceType': 'agentGenerated',
+      'color': _defaultEventColor,
+      'sourceType': _defaultSourceType,
     };
   }
 
@@ -93,17 +101,19 @@ class ToolRegistry {
     Map<String, dynamic> args,
   ) {
     final tool = _tools[toolName];
-    if (tool == null)
+    if (tool == null) {
       return ToolValidationResult(
         ok: false,
         error: 'Tool not found: $toolName',
       );
+    }
 
     final required = tool.parameters['required'] as List<dynamic>? ?? [];
     final missing = <String>[];
     for (final field in required) {
-      if (!args.containsKey(field) || args[field] == null)
+      if (!args.containsKey(field) || args[field] == null) {
         missing.add(field as String);
+      }
     }
 
     if (missing.isNotEmpty) {