fix(store): add None check for _task in AsyncRedisStore.__aexit__

Fixes type error where _task could be None but was being awaited without
proper type guard. Added explicit None check before calling .done() and
awaiting the task.

Resolves mypy error: Incompatible types in 'await' (actual type 'Task[Any] | None',
expected type 'Awaitable[Any]')

Author: bsbodden

langgraph/store/redis/aio.py

@@ -365,7 +365,7 @@ async def __aexit__(
     ) -> None:
         """Async context manager exit."""
         # Cancel the background task created by AsyncBatchedBaseStore
-        if hasattr(self, "_task") and not self._task.done():
+        if hasattr(self, "_task") and self._task is not None and not self._task.done():
             self._task.cancel()
             try:
                 await self._task