Cancel an in-flight job (user-requested). Transitions the job to cancelled and logs the cancellation (real-time AppSync delta); any kind-specific cleanup (e.g. releasing a slide lock) is applied by a subscriber on the terminal transition. Idempotent: a job that already reached a terminal state is returned unchanged (cancelled:false). The worker's in-flight work, if any, is left to finish and its later callback no-ops.