Send a test webhook

Tests every delivery channel this Monitor actually uses and reports each one under channels. A configured webhook receives the first attempt of a signed monitor.test event, synchronously; a retryable failure persists retry_at for the retry worker. Configured email receives a real test message at the same address a triggered run would use, from the same sender — so a suppressed or bouncing address is diagnosed in seconds instead of after a full cadence. Email tests are rate-limited per organization and answer channels.email.retry_after_seconds when refused. The test creates NO Monitor run: nothing is evaluated, no matches are marked as seen, and nothing appears in GET /api/v2/monitors/{id}/runs. The top-level delivered, reason, error, status and retry_at describe the webhook when one is configured and the email otherwise, so an email-only Monitor reports its real delivery rather than not_configured. Not refused on a plan without webhook entitlement when the Monitor has no webhook. Takes no query parameters and no body.