Write unit and integration tests for pause state machine

The state machine should be modeled as a pure function or a class method that accepts the current PeerMentorStatus and a requested transition, returning a Result/Either type rather than throwing exceptions — this makes branch coverage measurement straightforward. When mocking the repository, stub both the success path and failure path (e.g., Supabase network error) to ensure error propagation is tested. For the expired_cert transition, ensure the test simulates the certification expiry event as an input to the service rather than relying on a timer, since timers are difficult to test reliably. Use mockito's captor or argument matchers to assert the exact PauseRecord payload written to the repository.

Avoid testing UI or BLoC layer here — those belong in widget and integration tests.

Unit tests only — no widget or integration tests in this task. Use flutter_test with mockito or mocktail for mocking PauseManagementRepository and PauseNotificationService. Organize tests in groups: 'valid transitions', 'invalid transition guards', 'notification side effects', 'repository persistence'. Use setUp() to construct a fresh PauseManagementService with mocks before each test.

Use verify() / verifyNever() to assert mock interactions. Run flutter test --coverage and confirm coverage report shows ≥90% branch coverage on the service file. Parameterize transition tests where possible to avoid duplication.