Terminate the process associated with ExecutionId. The service looks up the execution record, resolves ProcessId and kills that process, then records Terminated by user with a failed result. A waiting execution without a live associated process may fail; this is not a general guarantee that any queued job can be removed. This GET changes execution state. Already rendered or delivered outputs are not undone. The recorded result has ErrorMessage="Execution cancelled by user", ErrorNumber=-102562911 and Result=false. Poll status after cancellation to reconcile the result.