Stop a widget draft request the user no longer wants

Records that the user stopped the draft request whose requestId the frontend sent. Aborting the fetch is not enough: Cloud Run does not pass an HTTP/1.1 client disconnect on to the container, so the draft would finish and be charged. A stopped draft is not returned and not charged (its token usage is recorded as failed / CLIENT_CANCELED). Safe to send more than once, and before or after the draft finishes; a Stop that arrives while the draft's charge is being written may be too late. Requires org.edit_dashboard.