feat(content_management): implement reset filter functionality

- Add event handler for FilterDialogReset event
- Implement _onFilterDialogReset method to reset filter selections
- Update FilterDialogBloc to handle reset functionality

Author: fulleni

lib/content_management/widgets/filter_dialog/bloc/filter_dialog_bloc.dart

@@ -68,6 +68,7 @@ class FilterDialogBloc extends Bloc<FilterDialogEvent, FilterDialogState> {
     on<FilterDialogHeadquartersCountryIdsChanged>(
       _onFilterDialogHeadquartersCountryIdsChanged,
     );
+    on<FilterDialogReset>(_onFilterDialogReset);
   }
 
   final DataRepository<Source> _sourcesRepository;
@@ -241,4 +242,12 @@ class FilterDialogBloc extends Bloc<FilterDialogEvent, FilterDialogState> {
   ) {
     emit(state.copyWith(selectedHeadquartersCountryIds: event.countryIds));
   }
+
+  /// Resets all temporary filter selections in the dialog to their initial state.
+  void _onFilterDialogReset(
+    FilterDialogReset event,
+    Emitter<FilterDialogState> emit,
+  ) {
+    emit(FilterDialogState(activeTab: state.activeTab));
+  }
 }