feat(消息通知): 添加对markdown格式消息的支持

新增send_markdown函数用于发送富文本格式消息，替换原有send_text调用以支持代码块、标题等格式内容显示。自动分割长消息为多个卡片发送，提升消息展示效果。

router/rpc.py

@@ -96,10 +96,10 @@ async def handle_task_complete(msg: TaskComplete) -> None:
     """Handle a task completion notification from a host client."""
     logger.info("Task %s completed for user %s", msg.task_id, msg.user_id)
 
-    from bot.feishu import send_text
+    from bot.feishu import send_markdown
 
     try:
-        await send_text(msg.chat_id, "chat_id", msg.result)
+        await send_markdown(msg.chat_id, "chat_id", msg.result)
     except Exception as e:
         logger.error("Failed to send task completion notification: %s", e)