feat: add cache expiration support to HydratedBloc

[meylis1998]

Summary

Adds optional cache expiration support to HydratedBloc and HydratedCubit to automatically clear cached state after a specified duration.

Changes

• Added optional expirationDuration parameter to HydratedBloc and HydratedCubit constructors
• Cached state with expiration wraps data with timestamp metadata
• Expired state returns default state and is automatically cleared from storage
• Fully backward compatible - existing blocs work without changes
• Handles legacy data gracefully (data without timestamp wrapper)

Storage Format

When expiration is enabled:

{
  "__hydrated_state__": { "value": 42 },
  "__hydrated_timestamp__": 1234567890
}

When expiration is disabled (default/legacy):

{ "value": 42 }

Example Usage

class CounterCubit extends HydratedCubit<int> {
  CounterCubit()
      : super(
          0,
          expirationDuration: const Duration(days: 7),
        );

  void increment() => emit(state + 1);

  @override
  int fromJson(Map<String, dynamic> json) => json['value'] as int;

  @override
  Map<String, int> toJson(int state) => {'value': state};
}

Test Coverage

• 12 new tests covering expiration logic, edge cases, and backward compatibility
• All 132 tests passing
• 100% test coverage for new code

Checklist

• Code formatted with dart format .
• Code analyzed with dart analyze --fatal-infos --fatal-warnings .
• All tests passing
• Documentation updated (README.md)
• Backward compatible

Fixes #4381