feat(agents): Improve handling of withLangGraph annotations

[pokey]

When passing withLangGraph as the stateSchema either directly to createAgent or to middleware, defaults are not properly detected

Note that I was not able to get the type checking to work properly here, so I'm leaving this as draft. See the tests and the type errors in CI

I would be tempted to have langgraph make types optional if a default is passed or if there is a reducer, which tries to apply a default, but maybe that wasn't done for a reason

[changeset-bot]

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