Delete a message
Permanently deletes a single message from the specified inbox by its message ID.
**Path parameters:**
- `:inbox` — URL-encoded inbox address (e.g., `test%40domain.com`)
- `:id` — Message ID returned by `GET /v1/inboxes/:inbox/messages`
**Behaviour:**
- Returns `204 No Content` on success — no response body.
- Returns `404` if the message or inbox does not exist.
- Deletion is immediate and irreversible.
**Plan:** All plans (Free and above).