Request cancellation and return {run_id, cancelling, state, phase}; the call is idempotent.

Costs nothing quota-wise. Call only on an explicit cancellation intent; phase pre_mine means quota was not spent, while post_mine remains billed. Recover from run_not_found by calling GET /v1/runs. For rate_limited or service_unavailable, wait retry_after_sec and retry once, then confirm terminal state with GET /v1/runs/{run_id}/progress.