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>

api/apps/document_app.py

@@ -240,9 +240,9 @@ async def create():
 @manager.route("/list", methods=["POST"])  # noqa: F821
 @login_required
 async def list_docs():
-    kb_id = request.args.get("kb_id")
+    kb_id = request.args.get("id")
     if not kb_id:
-        return get_json_result(data=False, message='Lack of "KB ID"', code=RetCode.ARGUMENT_ERROR)
+        return get_json_result(data=False, message='Dataset ID is required for listing files.', code=RetCode.ARGUMENT_ERROR)
     tenants = UserTenantService.query(user_id=current_user.id)
     for tenant in tenants:
         if KnowledgebaseService.query(tenant_id=tenant.tenant_id, id=kb_id):