Graph trajectory extraction does not handle `Command`/`Send` API

Specifically, `extract_langgraph_trajectory_from_snapshots` fails to detect a checkpoint of a node that used `Command`/`Send` to go to another node (such as with[ `langgraph_supervisor`](https://github.com/langchain-ai/langgraph-supervisor-py/blob/4ff497c1b04bc5038d51b58cd8399b26ddc28a5d/langgraph_supervisor/handoff.py#L106)).

This is probably because the `StateSnapshot` list does not have an `__end__` node checkpoint which fulfills the criteria of the function.

Start with extract_langgraph_trajectory_from_snapshots and inspect how its StateSnapshot list identifies node checkpoints. Compare that behavior with the linked langgraph_supervisor handoff.py Command/Send example. Done means the trajectory extractor handles this checkpoint pattern and has regression coverage for it.

Written by the indexing model from the issue text.