fix: use AsyncMock for concurrent test consistency

Address review feedback from greptile — use new_callable=AsyncMock
on the concurrent test's patch.object to ensure the mock is properly
typed as async, even though side_effect already handles the coroutine.

tests/test_litellm/proxy/test_aiohttp_session_recovery.py

@@ -167,6 +167,7 @@ async def test_add_shared_session_concurrent_recreation_uses_lock():
         with patch.object(
             proxy_server_module,
             "_initialize_shared_aiohttp_session",
+            new_callable=AsyncMock,
             side_effect=mock_init,
         ):
             # Launch 5 concurrent calls