fix(scheduler): split ImageCleanupTask prune (idempotent) from docuum launch

[jinbai340997]

Summary

• Extract _launch_docuum() (idempotent-gated) and _run_prune() (always runs)
• Override run_on_worker() to always call prune, then conditionally launch docuum
• Add tests covering both code paths and the regression scenario

Test plan

• uv run pytest tests/unit/admin/scheduler/test_image_cleanup_task.py -v PASS
• Pre-prod ops_jobs trigger ({"tasks": ["image_cleanup"]}) — prune output observed in admin log on every run regardless of
  docuum state
  fixes [Bug] ImageCleanupTask gates idempotent prune behind non-idempotent docuum launch #1022