Test cancelled notification, no processor, success, and failure paths. Coverage: 50.0% → 80.0%
Test early return for empty alertIDs and happy path with empty queue. Coverage: 62.9% → 65.7%
Test coverage for calculateBackoff (exponential backoff calculation), NotificationQueueStatus constants, and QueuedNotification struct fields. 15 test cases covering backoff timing, cap behavior, and struct defaults.