fix(tests): skip langmem tests on Python 3.10

Add Python version check to skip langmem serialization tests on Python < 3.11.
The langmem library uses typing.NotRequired which was introduced in Python 3.11,
causing import failures on Python 3.10 in CI.

Tests will:
- Skip on Python 3.10 (379 tests run)
- Run on Python 3.11+ (382 tests run)

Fixes CI test collection failures on Python 3.10.

Author: bsbodden

tests/test_langmem_serialization.py

@@ -1,10 +1,18 @@
 """Integration tests for langmem object serialization with JsonPlusRedisSerializer."""
 
+import sys
+
 import pytest
-from langmem.short_term.summarization import RunningSummary
 
 from langgraph.checkpoint.redis.jsonplus_redis import JsonPlusRedisSerializer
 
+# langmem requires Python 3.11+ (uses typing.NotRequired)
+# Use importorskip to skip the entire module if langmem can't be imported
+RunningSummary = pytest.importorskip(
+    "langmem.short_term.summarization",
+    reason="langmem requires Python 3.11+ (uses typing.NotRequired)",
+).RunningSummary
+
 
 class TestLangmemSerialization:
     """Test that langmem objects are properly serialized and deserialized."""