Send a synchronous test alert to a recipient (issue

Dispatches a [TEST] alert (email), a signed event: "monitor.test" POST (webhook), an unsigned 🧪 TEST Block Kit message (slack, issue #359), or a PagerDuty Events API v2 trigger-then-resolve pair (pagerduty, issue #363) directly — bypasses the webhook_deliveries outbox entirely, since there is no real monitor behind a test send. Rate-limited to 1 per recipient per minute. A pending or disabled recipient returns 409. For type=slack, a non-2xx response — 3xx included — is reported as status="failed" with response_code and error set to Slack's own response body (e.g. "no_service"); only a transport failure returns the top-level 502. For type=pagerduty, the trigger (severity info) is followed by an immediate resolve under the identical dedup_key, ONLY if the trigger's response was 2xx; a successful pair reports status="accepted" (never "delivered", since PagerDuty's Events API is asynchronous and two 2xx responses do not prove the test incident actually closed) with message carrying that caveat. Any failure — a non-2xx trigger, a transport error on either request, or a non-2xx resolve — reports status="failed"; every failure except a cleanly-rejected trigger also carries a warning in error that a test incident may remain open in PagerDuty. No PagerDuty response body is ever read or returned; error/message always come from fixed outcome text.