fix(eval): cancel queued provider-eval jobs when a standalone evaluation is deleted

[tryeverything24]

What

Deleting a standalone provider evaluation from the admin UI now also cancels its
still-queued job, so a "deleted" eval can no longer be picked up and executed by a
worker.

Why

DELETE /api/admin/evaluations/{id} soft-deletes a standalone provider evaluation by
setting deleted_at, but it left the evaluation's provider_eval row in job_queues
untouched. The worker's poll query (worker.process_once) only filters on
JobQueue.status == "queued" and never looks at EvaluationRun.deleted_at, so a
provider eval that was deleted before it ran would still be claimed, executed via
evaluate_provider_route(...), and charged real provider spend — the exact outcome an
operator is trying to avoid by deleting it. The run then completes while remaining
hidden from /api/provider-benchmarks (which filters on deleted_at), so the wasted
cost is also invisible.

The submission delete path already guards against this: DELETE /api/admin/submissions/{id}
calls cancel_submission_jobs(...). Standalone-evaluation delete was simply missing the
equivalent step.

Change

• Add cancel_evaluation_jobs(session, evaluation_id, reason=...) in
  services/queue.py, mirroring cancel_submission_jobs: it flips any queued
provider_eval job for that evaluation id to cancelled (a running job is left
  alone, matching the submission path).
• Call it from admin_delete_evaluation right after setting deleted_at, before the
  commit, so the soft-delete and the cancellation land in one transaction.

Revoke behaviour is unchanged; only the queued job is affected, and only on delete.

Tests

Adds validator/tests/test_delete_evaluation.py:

• deleting a standalone eval cancels its queued provider_eval job (RED before the fix:
  the job stayed queued),
• an unrelated evaluation's queued job is untouched,
• deleting a submission-backed evaluation is still rejected with 400.

Verification

• python -m pytest -q (validator suite) green against Postgres 16.
• ruff check . clean in validator/.