Donor acknowledges receipt of an open proxy claim, transitioning it
from open to pending_acknowledgment. Only applicable when
role = donor — requests against claimer-side rows return 400.

The provider call is synchronous against the simulator; on success
the response returns status = pending_acknowledgment and the claim
is finalised to waiting_resolution asynchronously by the inbound
proxy_claim.acknowledgment_completed event. On provider rejection
the response returns 200 with status = acknowledgment_rejected and
a populated status_details (rejections are stored business states,
not HTTP errors).