Raise exception if sync method on async checkpointer is called from main thread

- The thread running the event loop can block waiting for a coro to run, only background threads can

libs/checkpoint-sqlite/langgraph/checkpoint/sqlite/aio.py

@@ -153,6 +153,18 @@ class AsyncSqliteSaver(BaseCheckpointSaver[str]):
         Returns:
             Optional[CheckpointTuple]: The retrieved checkpoint tuple, or None if no matching checkpoint was found.
         """
+        try:
+            # check if we are in the main thread, only bg threads can block
+            # we don't check in other methods to avoid the overhead
+            if asyncio.get_running_loop() is self.loop:
+                raise asyncio.InvalidStateError(
+                    "Synchronous calls to AsyncSqliteSaver are only allowed from a "
+                    "different thread. From the main thread, use the async interface."
+                    "For example, use `await checkpointer.aget_tuple(...)` or `await "
+                    "graph.ainvoke(...)`."
+                )
+        except RuntimeError:
+            pass
         return asyncio.run_coroutine_threadsafe(
             self.aget_tuple(config), self.loop
         ).result()