fix(content_management): prevent multiple concurrent country load more requests

- Add countriesIsLoadingMore flag to state
- Check countriesIsLoadingMore before making new load more request
- Update state with countriesIsLoadingMore: true at the start of the request
- Reset countriesIsLoadingMore to false in both success and error cases

Author: fulleni

lib/content_management/bloc/create_headline/create_headline_bloc.dart

@@ -230,7 +230,9 @@ class CreateHeadlineBloc
     CreateHeadlineLoadMoreCountriesRequested event,
     Emitter<CreateHeadlineState> emit,
   ) async {
-    if (!state.countriesHasMore) return;
+    if (!state.countriesHasMore || state.countriesIsLoadingMore) return;
+
+    emit(state.copyWith(countriesIsLoadingMore: true));
 
     try {
       final countriesResponse = await _countriesRepository.readAll(
@@ -248,15 +250,23 @@ class CreateHeadlineBloc
           countries: List.of(state.countries)..addAll(countriesResponse.items),
           countriesCursor: countriesResponse.cursor,
           countriesHasMore: countriesResponse.hasMore,
+          countriesIsLoadingMore: false,
         ),
       );
     } on HttpException catch (e) {
-      emit(state.copyWith(status: CreateHeadlineStatus.failure, exception: e));
+      emit(
+        state.copyWith(
+          status: CreateHeadlineStatus.failure,
+          exception: e,
+          countriesIsLoadingMore: false,
+        ),
+      );
     } catch (e) {
       emit(
         state.copyWith(
           status: CreateHeadlineStatus.failure,
           exception: UnknownException('An unexpected error occurred: $e'),
+          countriesIsLoadingMore: false,
         ),
       );
     }