Send a test event

Queue ONE test delivery to the subscription's endpoint: the last delivered payload of this subscription if there is one, else a synthetic sample built from the catalogue and the field registry — always with `test: true` in the envelope, signed like a real event. No record cost. Returns match_id and delivery_id; list_subscription_matches shows the HTTP outcome. Test deliveries retry at most 3 times and never trip the endpoint's breaker.