API to cancel/delete scheduled broadcasts

We use the Broadcast API with scheduled_at to schedule WhatsApp reminders for individual users (e.g., 1 hour before a demo, 10 minutes before).

When a user cancels or reschedules their booking, we need to cancel the pending scheduled broadcasts via API.

Currently:

• POST /message/queues/purge with broadcast_id returns {"purged": true} but the message still delivers at the scheduled time

• DELETE /queue/purge also returns 204 but doesn't cancel delivery

• No other endpoint exists to cancel a scheduled broadcast

The only way to cancel is manually via Bulk Messages > Scheduled in the dashboard.

Requested: An API endpoint (e.g., DELETE /message/broadcast/{broadcast_id}) to cancel a scheduled broadcast before it's delivered.

This is critical for any booking/appointment reminder use case where users can cancel or reschedule.