Request cancellation of a Robot session turn

Create the singleton cancellation request for a queued, running, or blocked Robot turn. The operation is idempotent and asynchronous: accepted cancellation is reflected by a `turn_cancelled` event on the session stream. Cancelling a turn does not remove other queued messages, which remain eligible for later turns.

curl -X PUT "https://example.com/robots/sessions/cc5lnd2s1s4652adtu50/turns/cc5lnd2s1s4652adtu50/cancellation"

{
  "type": "string",
  "title": "string",
  "detail": "string",
  "trace_id": "string",
  "metadata": {}
}