feat: Require messages to start with /ai

Co-authored-by: jxxghp <jxxghp@qq.com>

app/chain/message.py

@@ -943,6 +943,16 @@ class MessageChain(ChainBase):
                 return
 
             # 提取用户消息
+            if not text.startswith("/ai"):
+                self.post_message(Notification(
+                    channel=channel,
+                    source=source,
+                    userid=userid,
+                    username=username,
+                    title="消息格式错误，请以 /ai 开头"
+                ))
+                return
+            
             user_message = text[3:].strip()  # 移除 "/ai" 前缀
             if not user_message:
                 self.post_message(Notification(