List conversation messages

Calls off a message you scheduled with `scheduled_at`, before it is released for sending. The message moves to `CANCELLED`, which is terminal, and it is never sent. **Only a SCHEDULED message can be cancelled.** A message that has already been released — queued, in flight, or finished — returns 409. The release sweeper runs continuously, so a cancellation sent within about a minute of the scheduled instant can lose that race and be refused; cancel with time to spare. **One message per call.** A send to many recipients, or on several channels, becomes one message per recipient × channel, each with its own id; cancel the ones you want stopped. There is no bulk cancellation. **Cancelling is free**, and nothing was charged for the hold in the first place — a scheduled message is only priced and billed at release. **A cancellation fires `message.cancelled`** on your webhook, exactly once, carrying the `scheduled_at` that was called off so a consumer can un-record the send it was expecting. Cancelling the same message again returns this same response and fires no second event. `Idempotency-Key` is honoured. To send after cancelling, send a new message. The response is the message itself, in the same shape `GET /v3/messages/{id}` returns, so the `status` you read back is the one now stored on the row, and `scheduled_at` is the time it would have gone out. `GET /v3/messages/{id}` keeps reporting both afterwards. `events` is the message lifecycle as it stood immediately before the cancellation: the CANCELLED entry is written asynchronously and lands a moment later, so read `status` for the outcome. If you need the entry itself, re-fetch the message after a few seconds — `GET /v3/messages/{id}` is cached briefly, so an immediate re-fetch can still show the pre-cancellation state.