Answer MCP elicitation requests with a LangGraph interrupt.
A server needing input mid-call returns an InputRequiredResult and expects the
tools/call to be retried with answers. This module drives that loop, sourcing
each answer from interrupt() so the human reviewing the agent answers too.
We drive the loop here rather than via the SDK's run_input_required_driver
because that driver answers from callbacks run concurrently in a task group:
LangGraph matches resume values to interrupt() calls by order, so concurrent
firing scrambles the matching, and FastMCP would swallow the GraphInterrupt as
an MCP error. Calling interrupt() from this frame keeps one per round.
Only elicitation is answered. Sampling, roots, and continuation rounds are refused rather than half-served.
