feat: add tests for dunning email sequences with A/B testing (#956)

- Add backend/services/notification/__tests__/dunningEmailSequences.test.ts
  with comprehensive test coverage for DunningEmailSequenceService
- Tests cover: variant management (create/update/delete), sequence
  lifecycle, weighted A/B assignment and result tracking, delivery
  logging, and deliverability metrics (open/click rates)
- Validates the existing dunningEmailSequences.ts implementation meets
  all acceptance criteria

Technical scope: backend/services/notification/, backend/services/billing/dunning.ts

Author: DevSolex