feat(content_management): add pagination events to source blocs

Introduces new events to `CreateSourceBloc` and `EditSourceBloc`
to support paginated and searchable dropdowns for both countries
and languages.

- `CountrySearchChanged` / `LanguageSearchChanged`: Dispatched when the
  user types in the search field.
- `LoadMoreCountriesRequested` / `LoadMoreLanguagesRequested`: Dispatched
  when the user scrolls to the end of the list, requesting the next page.

Author: fulleni

lib/content_management/bloc/create_source/create_source_event.dart

@@ -75,3 +75,29 @@ final class CreateSourceStatusChanged extends CreateSourceEvent {
 final class CreateSourceSubmitted extends CreateSourceEvent {
   const CreateSourceSubmitted();
 }
+
+/// Event for when the country search term is changed.
+final class CreateSourceCountrySearchChanged extends CreateSourceEvent {
+  const CreateSourceCountrySearchChanged(this.searchTerm);
+  final String searchTerm;
+  @override
+  List<Object?> get props => [searchTerm];
+}
+
+/// Event to request loading more countries.
+final class CreateSourceLoadMoreCountriesRequested extends CreateSourceEvent {
+  const CreateSourceLoadMoreCountriesRequested();
+}
+
+/// Event for when the language search term is changed.
+final class CreateSourceLanguageSearchChanged extends CreateSourceEvent {
+  const CreateSourceLanguageSearchChanged(this.searchTerm);
+  final String searchTerm;
+  @override
+  List<Object?> get props => [searchTerm];
+}
+
+/// Event to request loading more languages.
+final class CreateSourceLoadMoreLanguagesRequested extends CreateSourceEvent {
+  const CreateSourceLoadMoreLanguagesRequested();
+}

lib/content_management/bloc/edit_source/edit_source_event.dart

@@ -86,3 +86,29 @@ final class EditSourceStatusChanged extends EditSourceEvent {
 final class EditSourceSubmitted extends EditSourceEvent {
   const EditSourceSubmitted();
 }
+
+/// Event for when the country search term is changed.
+final class EditSourceCountrySearchChanged extends EditSourceEvent {
+  const EditSourceCountrySearchChanged(this.searchTerm);
+  final String searchTerm;
+  @override
+  List<Object?> get props => [searchTerm];
+}
+
+/// Event to request loading more countries.
+final class EditSourceLoadMoreCountriesRequested extends EditSourceEvent {
+  const EditSourceLoadMoreCountriesRequested();
+}
+
+/// Event for when the language search term is changed.
+final class EditSourceLanguageSearchChanged extends EditSourceEvent {
+  const EditSourceLanguageSearchChanged(this.searchTerm);
+  final String searchTerm;
+  @override
+  List<Object?> get props => [searchTerm];
+}
+
+/// Event to request loading more languages.
+final class EditSourceLoadMoreLanguagesRequested extends EditSourceEvent {
+  const EditSourceLoadMoreLanguagesRequested();
+}