chore: fix flaky unit test and add proper shutdown for file batches (#3725)

# What does this PR do?
Have been running into flaky unit test failures:
5217035494
Fixing below
1. Shutting down properly by cancelling any stale file batches tasks
running in background.
2. Also, use unique_kvstore_config, so the test dont use same db path
and maintain test isolation
## Test Plan
Ran unit test locally and CI

llama_stack/providers/utils/memory/openai_vector_store_mixin.py

@@ -293,6 +293,19 @@ class OpenAIVectorStoreMixin(ABC):
         await self._resume_incomplete_batches()
         self._last_file_batch_cleanup_time = 0
 
+    async def shutdown(self) -> None:
+        """Clean up mixin resources including background tasks."""
+        # Cancel any running file batch tasks gracefully
+        if hasattr(self, "_file_batch_tasks"):
+            tasks_to_cancel = list(self._file_batch_tasks.items())
+            for _, task in tasks_to_cancel:
+                if not task.done():
+                    task.cancel()
+                    try:
+                        await task
+                    except asyncio.CancelledError:
+                        pass
+
     @abstractmethod
     async def delete_chunks(self, store_id: str, chunks_for_deletion: list[ChunkForDeletion]) -> None:
         """Delete chunks from a vector store."""