Refund an MPP deposit intent (system admin, dry_run by default)

Returns the exact call (forwarder refund, after a factory deploy if needed) without sending anything unless dry_run is explicitly false. Refused unless the intent is underpaid, overpaid, parked or expired. Idempotent per intent. mode=reserve reserves the intent and returns the plan with action_id. A non-dry refund answers 409 refund_to_contract when the refund address has code or is a forwarder.