Triggers a synchronous test delivery to the webhook's configured URL using a sample payload with 'test': true. Returns immediate feedback including the HTTP status code and round-trip duration in milliseconds. The target's response body is not reflected in this response; it is recorded on the webhook's delivery history instead. 'response_body' is only present on request-level failures (timeout, connection error) and carries a diagnostic message, not target content. Only the webhook owner can trigger a test delivery. Uses a 10-second timeout.