Complete a proxy claim. Only the claimer side can call this endpoint
(role = claimer). Completion rules depend on the claim type:

• type=portability — completable once status = confirmed, regardless of deadline.
• type=ownership — completable once status = confirmed and completion_period_end has passed.

Completion is async at the scheme. On success the response returns
status = pending_completion: the request was accepted and is awaiting
the scheme to finalise it. The claim is flipped to completed — and the
new active proxy key created and linked via proxy_key_id — by the
proxy_claim.completion_completed inbound event. On provider rejection
the response returns 200 with status = completion_rejected and a
populated status_details (rejections are stored business states, not
HTTP errors).