Cancel a send

Cancels a scheduled or queued send before it goes out, or STOPS any in-flight campaign send. A campaign in sending or paused is canceled the same way the in-app Stop button does it: the remaining recipients are never delivered while already-sent ones stay sent (every shape re-checks liveness at regular points mid-flight — smart per time bucket, gradual and plain blast at bounded chunk intervals — so a small tail may still deliver after the cancel lands). Idempotent — a send already canceled returns 200. A send already sent or failed, or a non-campaign (automation) send, returns 409 SEND_NOT_CANCELLABLE. Brand-scoped: an unknown / cross-brand sendId is 404.