Regression tests pinning the zero-deletion safety boundary

[rfsbraz]

Description

Nothing in the test suite asserts the single most important property of a deletion tool: when inputs are degraded, deleterr deletes NOTHING.

There is no test that empty/failed Tautulli watch activity yields zero deletions (#287), none that an mdblist fetch failure skips the library (#288), and the Trakt fail-safe behavior only recently gained coverage. These paths guard irreversible actions and currently rely on review vigilance instead of CI.

Proposed Tests

• Empty Tautulli activity => zero deletions, library flagged, loud log
• Partial/failed history pagination => fail safe, zero deletions
• mdblist/Trakt fetch raising => library skipped, zero deletions, counted as failed
• Dry run => state file byte-identical afterwards (Dry-run mutates the persistent state file #291)
• A shared "no deletions happened" assertion helper so future safety fixes land with their boundary pinned

Notes

Best landed alongside the fixes for #287/#288/#291, but the helper and the already-safe paths can go in first - they document today's intended safety contract.