Return a human-readable message if the client rejected an interrupt.
Scans for :class:MCPApprovalResponse items whose
approval_request_id matches a pending interrupt and whose
approve is False. The first match wins; subsequent rejections
are ignored.
The host's :meth:handle_create calls this before attempting to
resume so a rejection short-circuits the turn into
response.failed instead of being silently dropped.
Resolved input items from the request.
Pending interrupts on the graph's checkpointed state.
