fix: address second-round review feedback on bot identity PR

src/chat/brain_chat/PFC/message_sender.py

@@ -44,8 +44,10 @@ class DirectMessageSender:
             # 获取麦麦的信息
             bot_user_id = get_bot_account(chat_stream.platform)
             if not bot_user_id:
-                logger.warning(f"[私聊][{self.private_name}]平台 {chat_stream.platform} 未配置机器人账号，发送消息时回退到 QQ 账号")
-                bot_user_id = str(getattr(global_config.bot, "qq_account", "")).strip()
+                bot_user_id = get_bot_account("qq")
+            if not bot_user_id:
+                logger.error(f"[私聊][{self.private_name}]平台 {chat_stream.platform} 无可用机器人账号，无法发送消息")
+                raise RuntimeError("机器人账号未配置")
             bot_user_info = UserInfo(
                 user_id=bot_user_id,
                 user_nickname=global_config.bot.nickname,