feat(content): update create source page to use reactive dropdowns

Refactored the `create_source_page.dart` to use the new BLoC-aware `SearchableDropdownFormField` for both language and country selection.

- Replaced the old dropdown implementations with the new generic `SearchableDropdownFormField`.
- Injected the `CreateSourceBloc` instance into each widget.
- Provided extractor functions (`itemsExtractor`, `hasMoreExtractor`, `isLoadingExtractor`) to enable the dropdowns to reactively pull the correct data (languages or countries) from the BLoC's state.

This change ensures that both the language and country lists in their respective search dialogs update automatically when more items are loaded via pagination.

Author: fulleni

lib/content_management/view/create_source_page.dart

@@ -161,12 +161,15 @@ class _CreateSourceViewState extends State<_CreateSourceView> {
                           .add(CreateSourceUrlChanged(value)),
                     ),
                     const SizedBox(height: AppSpacing.lg),
-                    SearchableDropdownFormField<Language>(
+                    SearchableDropdownFormField<Language, CreateSourceBloc,
+                        CreateSourceState>(
                       labelText: l10n.language,
-                      items: state.languages,
+                      bloc: context.read<CreateSourceBloc>(),
                       initialValue: state.language,
-                      hasMore: state.languagesHasMore,
-                      isLoading: state.status == CreateSourceStatus.loading,
+                      itemsExtractor: (state) => state.languages,
+                      hasMoreExtractor: (state) => state.languagesHasMore,
+                      isLoadingExtractor: (state) =>
+                          state.status == CreateSourceStatus.loading,
                       onChanged: (value) => context
                           .read<CreateSourceBloc>()
                           .add(CreateSourceLanguageChanged(value)),
@@ -206,12 +209,15 @@ class _CreateSourceViewState extends State<_CreateSourceView> {
                           .add(CreateSourceTypeChanged(value)),
                     ),
                     const SizedBox(height: AppSpacing.lg),
-                    SearchableDropdownFormField<Country>(
+                    SearchableDropdownFormField<Country, CreateSourceBloc,
+                        CreateSourceState>(
                       labelText: l10n.headquarters,
-                      items: state.countries,
+                      bloc: context.read<CreateSourceBloc>(),
                       initialValue: state.headquarters,
-                      hasMore: state.countriesHasMore,
-                      isLoading: state.status == CreateSourceStatus.loading,
+                      itemsExtractor: (state) => state.countries,
+                      hasMoreExtractor: (state) => state.countriesHasMore,
+                      isLoadingExtractor: (state) =>
+                          state.status == CreateSourceStatus.loading,
                       onChanged: (value) => context
                           .read<CreateSourceBloc>()
                           .add(CreateSourceHeadquartersChanged(value)),