Cancel Simulation Run

Cancel an in-flight simulation run.

Cascades the cancel down to child test_results:

1. Set simulation_run.status = COMPLETED (the only terminal state after the SimulationRunStatus collapse). The fact that the user cancelled the run is captured by child test_result rows transitioning to CANCELLED, not by a distinct sim_run status.
2. For each child test_result still in PRE_RUNNING_STATUSES, call mark_cancelled_async() — guarded so it raises if a status snuck out of pre-running between our read and write.
3. Children that already started running (RUNNING / CONVERSATION_ENDED / EVALUATING) are LEFT ALONE — they keep their natural terminal status so dropped/completed calls still produce meaningful eval data.
4. Bump simulation_run.tests_cancelled by the cascade count.

The endpoint is idempotent for already-terminal sim runs (returns success=False with a message; doesn't try to re-cancel).