fix(risk_acceptance): reinstate findings when expiration date updated via API

[Jino-T]

[sc-13436]

Summary

• RiskAcceptanceSerializer.update() never called ra_helper.reinstate() when expiration_date changed, unlike the legacy Django view (engagement/views.py:1308-1310) which does so correctly.
• This caused two related bugs:
  1. Findings stay Active after extending an expired RA — when a user edits the expiration date from a past date to a future date via the API (Edit Risk Acceptance form in the UI), reinstate() is never called, so findings remain active=True / risk_accepted=False instead of being set back to inactive/risk-accepted.
  2. Findings stay Inactive on subsequent expiry cycles — because reinstate() was never called, expiration_date_handled was never cleared. The Celery expiration task queries with expiration_date_handled__isnull=True, so the RA is permanently excluded from every future expiry run and findings are never reactivated when the RA expires again.

Fix

Capture old_expiration_date before super().update(), then call ra_helper.reinstate() when the date changes — matching the logic that already exists in the legacy Django view.

old_expiration_date = self.instance.expiration_date
instance = super().update(instance, validated_data)
...
if instance.expiration_date != old_expiration_date:
    ra_helper.reinstate(instance, old_expiration_date)

reinstate() handles both outcomes internally: it reinstates findings (only when expiration_date_handled is set) and unconditionally clears expiration_date_handled and expiration_date_warned, making the RA eligible for the next Celery expiry cycle.

Test plan

• Edit a Risk Acceptance with an expired date to a future date via the API — verify findings become active=False / risk_accepted=True
• Confirm expiration_date_handled is None after the update — verify the Celery task will pick it up on the next expiry cycle
• Edit a fresh (never-expired) RA's date — verify findings are unchanged and expiration_date_handled stays None

🤖 Generated with Claude Code

[mtesauro]

Approved