Context

Describe the background behind the problem.

During work to disprove claims that our performance was slow, I used a test suggested by the LangGraph team (called fanout-something-or-rather) and set up an environment to compare to in-memory, and more importantly, PostGres. The results are captured in INTPYTHON-637.

This task is to take the test created and add it to our test set, as it is an example of use of the higher-level graph api (as opposed to checkpointer or store). The code exists in langchain-mongodb/pull/149. The primary task is to remove the PostGres dependency.

Definition of done

Add existing test to our repo, and ensure it passes on ai-ml pipelines.