Fix: Added defaultRunInput to .actor/actor.json (Layer 3 of §226).
Symptom: Apify Store auto-QA runs with empty input hit Layer 2 fallback (DEFAULT_URL) instead of running with the defaultRunInput the schema advertises.
Fix: defaultRunInput mirrors schema defaults 1:1 (url, engine, commentId, raw), so empty-input runs now execute the schema-declared defaults.