maybe bug in _is_field_binop

[Hacky-DH]

In code here

https://github.com/langchain-ai/langgraph/blob/main/libs/langgraph/langgraph/graph/state.py#L770

if len(meta) >= 1 and callable(meta[-1]):
    sig = signature(meta[0])

I think this line should be sig = signature(meta[-1])
how about this?

[Hacky-DH]

fix here 363c6e2