Integrate validation and confirmation emission in BLoC

To integrate Riverpod from within a BLoC, pass a WidgetRef or Riverpod Ref into the bloc constructor — prefer a thin callback: `final void Function(DateTimeRange) onPeriodConfirmed` injected at construction. This avoids tight coupling between the BLoC and Riverpod internals and makes the bloc trivially testable without a ProviderScope. The widget that creates the bloc (via BlocProvider) captures the ref and passes the callback. Add PeriodConfirmed as a distinct state separate from PeriodSelectionLoaded — this makes it easy to listen only for confirmation without re-rendering the entire form.

In the BlocListener in the parent widget, check for PeriodConfirmed and call GoRouter.of(context).go('/bufdir-report/preview'). Clear validationErrorMessage by including it as a nullable field on PeriodSelectionLoaded and setting it to null in copyWith calls for SelectPreset and ChangeCustomRange handlers.

Unit tests with bloc_test and a ProviderContainer for Riverpod. Required test cases: (1) invalid range — start after end, verify PeriodSelectionLoaded emitted with validationErrorMessage set, Riverpod provider NOT updated; (2) range exceeds max — verify correct error message for that specific rule; (3) valid range — verify PeriodConfirmed emitted, confirmedReportPeriodProvider updated with correct DateTimeRange; (4) idempotency — confirm same valid range twice, verify no double-emission side effects; (5) state guard — dispatch ConfirmPeriod when state is PeriodSelectionInitial, verify PeriodSelectionError emitted; (6) error clear on range change — emit validation error, then dispatch SelectPreset, verify validationErrorMessage is null in next state. Use a mock ReportPeriodValidator that returns configurable ValidationResult.