Test Webhook

Send a one-off test delivery to a registered webhook (rate-limited to 10/min) to confirm it's wired up correctly. On failure the response carries reason and status plus hint (our sentence naming the fix, most often that the target's kind does not match its host) and detail (a bounded, sanitised copy of what the destination itself replied). Returns 404 webhook_not_found, or webhook_url_rejected if the URL fails re-validation (e.g. now resolves to a private address). Cost: Free per call.