Resume a payer's MPP deposit intent by refund address (service key, guest checkout)

Service-key only (the same callers as POST /service/fulfillments/{id}/mpp-deposit-intent). Read-only: returns the intent THIS payer (refund_address) already holds on the fulfillment, the same body the create answers, so a guest who closed the tab can find their deposit address again. Never creates an intent. Only the intent whose refund address the caller names can be found, so nothing about any other payer is revealed. The fulfillment must belong to X-Tenant-ID. 404 when the fulfillment is unknown, another tenant's, or has no intent for that refund address; 400 when refund_address is missing or not an address.