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.

On success the response returns status = completed with proxy_key_id populated — a new proxy key is created in active status. On provider rejection the response returns 200 with status = completion_rejected and a populated status_details (rejections are stored business states, not HTTP errors).