Cancel one or more contact messages that are still scheduled.

No billing impact: for API sends, payment is only captured at the real send time, so cancelling
a scheduled recipient before it is emitted never charges and never refunds.

Important notes:

• ids are the 12-digit friendly ids of the contact messages, as returned by bulk_send.
• All ids must currently be scheduled, with a scheduledAt still in the future, and belong to the
  authenticated sender/reseller. If any id fails these checks, nothing is cancelled and a 404 is returned.
• scheduledAt is required as part of the shared request schema (see scheduled-at-too-soon /
  scheduled-at-too-far in the 422 examples).