Tests: mark the ftplib import in the FTP listing test for Bandit

- The test raises a real ftplib.error_perm; mark the import with
  nosec B402 like the other FTP tests, so Bandit stops failing the PR

backend/tests/unit/services/test_ftp_list_timeout_state.py

@@ -7,7 +7,7 @@ started another one in the shared FTP pool.
 """
 
 import asyncio
-import ftplib
+import ftplib  # nosec B402 -- tests construct real ftplib error types
 import threading
 from unittest.mock import patch