fix: news图文消息url字段必填，添加默认值

企微appchat/send的news消息类型要求url必填，否则报errcode 41010

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/services/wechat_service.py

@@ -417,9 +417,11 @@ class WeChatService:
             return False
         try:
             access_token = await self._get_access_token()
-            article = {"title": title, "description": description}
-            if url:
-                article["url"] = url
+            article = {
+                "title": title,
+                "description": description,
+                "url": url or "https://work.weixin.qq.com",
+            }
             if picurl:
                 article["picurl"] = picurl