Fix: broken /api/v1/chats endpoint (#5785)

### What problem does this PR solve?

The `/api/v1/chats` API endpoint was broken, any GET request got the
following response:
```
{"code":100,"data":null,"message":"TypeError(\"'int' object is not callable\")"}
```

With this log ragflow-server side:

```
2025-03-07 14:36:26,297 ERROR    20 'int' object is not callable
Traceback (most recent call last):
  File "/ragflow/.venv/lib/python3.10/site-packages/flask/app.py", line 880, in full_dispatch_request
    rv = self.dispatch_request()
  File "/ragflow/.venv/lib/python3.10/site-packages/flask/app.py", line 865, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)  # type: ignore[no-any-return]
  File "/ragflow/api/utils/api_utils.py", line 303, in decorated_function
    return func(*args, **kwargs)
  File "/ragflow/api/apps/sdk/chat.py", line 323, in list_chat
    logging.WARN(f"Don't exist the kb {kb_id}")
TypeError: 'int' object is not callable
2025-03-07 14:36:26,298 INFO     20 172.18.0.6 - - [07/Mar/2025 14:36:26] "GET /api/v1/chats HTTP/1.1" 200 -
``` 
This was caused by the incorrect use of `logging.WARN` as a method (it's
a loglevel object), instead of the correct `logging.warning()` method.

This PR fixes that, and also rewrites the message to be grammaticaly
correct.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/sdk/chat.py

@@ -318,7 +318,7 @@ def list_chat(tenant_id):
         for kb_id in res["kb_ids"]:
             kb = KnowledgebaseService.query(id=kb_id)
             if not kb:
-                logging.WARN(f"Don't exist the kb {kb_id}")
+                logging.warning(f"The kb {kb_id} does not exist.")
                 continue
             kb_list.append(kb[0].to_json())
         del res["kb_ids"]