Complete or decline a refund reservation

Completes or declines a pending refund reservation. Completion verifies an existing Stripe refund by authenticated provider read before applying the exact credit, tax and gross adjustment, and converges on the recorded adjustment when that refund already settled the reservation; it never creates a provider refund. Decline releases the reserved credit without contacting the provider, and a reservation a Stripe refund superseded is refused as a state conflict. The Idempotency-Key makes either outcome converge across retries.