refactor: remove unused MagicMock import in test_dynamodb_store.py

Author: 5enxia

libs/langgraph-checkpoint-aws/tests/unit_tests/store/dynamodb/test_dynamodb_store.py

@@ -2,7 +2,7 @@
 
 import pytest
 from datetime import datetime, timezone
-from unittest.mock import Mock, MagicMock, patch
+from unittest.mock import Mock, patch
 from langgraph.store.base import (
     GetOp,
     Item,