Update a broadcast.

Setting SendDateTime to a future UTC date schedules the broadcast for sending and its status changes to Pending. An account may only have 5 broadcasts scheduled or sending at one time; scheduling beyond that is rejected until one finishes sending or is unscheduled. There is no limit on the number of drafts. Clearing SendDateTime unschedules the broadcast. Recipients can be added with EmailListUids and SegmentUids (see the create endpoint): on update these are merged onto the broadcast's existing recipients and only ever add — to remove recipients or replace the set, send RecipientData directly.

When changing Message.Body, omit Message.Design: it is regenerated from the new Body so the drag-and-drop editor stays in sync. Sending a Design that was captured from an earlier response alongside an edited Body would otherwise keep the stale design. A body that was composed into the API email layout keeps the layout on such updates without duplicating it — its content is re-composed into the layout — and a bare fragment is composed like on create. See the create endpoint for how Body is rendered (Liquid), when the layout applies, and the unsubscribe-token expectation when it does not.