Refactor imports and remove unused code in test_security.py

Cleaned up unused imports including `timezone` and `ExpiredSignatureError`. Streamlined the import structure to improve readability and maintainability. These changes do not affect functionality but enhance code clarity.

backend/tests/auth/test_security.py

@@ -1,6 +1,8 @@
+from datetime import timedelta, datetime
+
 import pytest
-from datetime import timedelta, datetime, timezone
-from jose import jwt, JWTError, ExpiredSignatureError
+from jose import jwt, JWTError
+
 from app.auth.security import (
     get_password_hash, verify_password,
     create_access_token, create_refresh_token,
@@ -69,7 +71,6 @@ def test_decode_token_missing_exp():
     assert str(exc_info.value) == "Malformed token. Missing required claim."
 
 
-
 def test_decode_token_missing_sub():
     # Create a token without the `sub` claim
     payload = {"exp": datetime.now().timestamp() + 60, "type": "access"}