Send a text or image message to the guest.

This endpoint is not idempotent and does not return a message ID. If a request times out, the outcome is unknown — Hostex cannot determine whether the channel has already delivered the message, and this uncertainty cannot be resolved on our side, so no idempotency guarantee is offered. If you receive an error, retry as appropriate — in our experience, a guest receiving a duplicate message is better than a guest receiving no message at all.