aensure_thread

Ensure the remote thread record exists before mutating state.

In the LangGraph dev server, checkpoint persistence and HTTP thread registration are separate. After a server restart, a thread may still have checkpointed state on disk while POST /threads/{id}/state returns 404 because the server has not yet materialized that thread in its live store.

This method performs the idempotent HTTP-side registration with if_exists='do_nothing' so callers that recovered state from persistence can safely follow up with aupdate_state.