fix(langchain): fix state in wrapToolCall and wrapModelCall

[christian-bromann]

wrapToolCall and wrapModelCall do not properly retain the state of the middleware. This patch fixes these issues and adds test coverage to ensure we don't regress.

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Latest commit: 022ebfa

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[dqbd]

Does the #stateManager properly persist in checkpointer?

[christian-bromann]

I don't think it needs to. It is just s small helper class to help merge the state between 4 nodes into a single agent state, e.g. if you change state in beforeModel it makes sure that afterModel gets these updates.