Fix list files (#13960)

### What problem does this PR solve?

As title.

### Type of change

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


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Standardized the query parameter used when listing documents so
listings behave consistently across the web and client interfaces.
* Clarified the error message shown when a required dataset ID is
missing to give clearer guidance to users.

* **Tests**
* Updated test coverage to reflect the standardized dataset identifier
usage.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

admin/client/ragflow_client.py

@@ -1685,7 +1685,7 @@ class RAGFlowClient:
             time.sleep(0.5)
 
     def _list_documents(self, dataset_name: str, dataset_id: str):
-        response = self.http_client.request("POST", f"/document/list?kb_id={dataset_id}", use_api_base=False,
+        response = self.http_client.request("POST", f"/document/list?id={dataset_id}", use_api_base=False,
                                             auth_kind="web")
         res_json = response.json()
         if response.status_code != 200: