Add execution-level tests for v2 sync endpoints and background worker

Tester coverage gaps addressed:
- 5 TestClient execution tests for GET poll endpoint: 404 expired job,
  403 wrong owner, completed with result, failed with error, processing
  excludes result
- 7 behavioral background worker tests: partial failure reporting,
  all-failed reporting, DG usage recording (enabled/disabled), job-dir
  cleanup on success, job-dir cleanup on failure

Total: 63 tests in test_sync_v2.py (was 52), 103 across both suites.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: beastoin