precursor to `prepare_call` PR, cleaning up existing logic w/ pre model
hook
* use one combined `AgentState` instead of the one w/ and w/o structured
response
* remove exposed pydantic agent state + loosen bounds on state type
* remove llm input messages pattern, should be made possible with
prepare_call
also
* some remaining test fixes in `langgraph` to adapt to new `model` node
name (used to be `agent`)
