Cancel in-progress DTMF collection

**Rate limit: Per command** · **Burst:** 4 · **Refill:** 0.8 req/s This DELETE has its own bucket. The POST is configured identically, but uses a separate bucket. Cancel the in-progress prompt-and-collect operation on this session. Interrupts the prompt/collect cycle; the resulting `digits.collected` carries `status: cancelled`. Applies to the most recently dispatched DTMF collection; with none in progress the request is accepted and has no effect. Residual latency: cancellation stops prompt audio promptly, but the cancelled command's terminal `digits.collected` (`status: cancelled`) does not fire until the in-progress collect phase ends on the media layer's own timers — it is not delivered the instant you call DELETE. Input entered after a cancel is discarded — reported on the cancelled command's `digits.collected` event, never delivered to a later command.