chore(agent-eval): fix MemorySaver version compatibility issue

[weekenthralling]

After langgraph-checkpoint>=2.0.5, MemorySaver introduced ExitStack to dynamically manage contexts. However, using the with keyword does not work as expected.

In MemorySaver, the__enter__method returns an ExitStack object, and ExitStack does not have a get_next_version method.

def __enter__(self) -> "MemorySaver":
        return self.stack.__enter__()

This leads to an error when calling graph.invoke, error message: "AttributeError: 'ExitStack' object has no attribute 'get_next_version'".

# AsyncPregelLoop
self.checkpointer_get_next_version = checkpointer.get_next_version