Draft and send a message reply on a Front conversation via the Front API

Steps

1. Call POST https://api2.frontapp.com/conversations/{conversation_id}/messages (scope messages:send) with a required body field plus optional to/cc/bcc, subject, author_id, and channel_id to send a reply appended to the conversation.
2. Expect a 202 Accepted response acknowledging the send request (not a delivery confirmation), or a 301 if the conversation has been merged.
3. To stage an editable draft instead of sending immediately, call POST https://api2.frontapp.com/conversations/{conversation_id}/drafts (scope drafts:write), which requires channel_id and body.
4. Set the draft's mode to private (default, visible only to the author) or shared (visible to all teammates with access to the conversation).
5. Attach files as multipart/form-data on either endpoint (max 25 MB per file per Front's Attachments docs), and optionally set signature_id or should_add_default_signature.
6. There is no dedicated 'send draft' endpoint — to actually send, compose and call Create Message or Create Message Reply directly; the drafts endpoint is intended for in-app teammate revision.