Send a test notification through the real dispatch path (notify())

curl --request POST \
  --url https://api-sandbox.featherhq.com/v1/notifications/types/{event_type}/test-send \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '{
  "payload": {}
}'

{
  "outcome": "<string>",
  "event_type": "<string>",
  "delivered": true,
  "send_results": [
    {
      "ok": true,
      "provider_message_id": "<string>",
      "error": "<string>",
      "transport": "<string>"
    }
  ]
}