fix(test): add AsyncMock to top-level imports in test_s3_v2

CI failed with NameError because AsyncMock was not imported at module
level — only inline in some test methods.

tests/test_litellm/integrations/test_s3_v2.py

@@ -1,6 +1,6 @@
 import asyncio
 from datetime import datetime
-from unittest.mock import MagicMock, patch
+from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest