fix(ads): correct ad placement across paginated content

- Update processedContentItemCount to only count Headline items in feed
- Fix ad placement issues in entity details, headlines feed, and search feed screens
- Improve accuracy of ad placement rules across paginated loads

Author: fulleni

lib/entity_details/bloc/entity_details_bloc.dart

@@ -212,7 +212,10 @@ class EntityDetailsBloc extends Bloc<EntityDetailsEvent, EntityDetailsState> {
         feedItems: headlineResponse.items,
         user: currentUser,
         adConfig: remoteConfig.adConfig,
-        processedContentItemCount: state.feedItems.length,
+        // Calculate the count of actual content items (headlines) already in the
+        // feed. This is crucial for the FeedDecoratorService to correctly apply
+        // ad placement rules across paginated loads.
+        processedContentItemCount: state.feedItems.whereType<Headline>().length,
       );
 
       emit(

lib/headlines-feed/bloc/headlines_feed_bloc.dart

@@ -113,7 +113,10 @@ class HeadlinesFeedBloc extends Bloc<HeadlinesFeedEvent, HeadlinesFeedState> {
         feedItems: headlineResponse.items,
         user: currentUser,
         adConfig: remoteConfig.adConfig,
-        processedContentItemCount: state.feedItems.length,
+        // Calculate the count of actual content items (headlines) already in the
+        // feed. This is crucial for the FeedDecoratorService to correctly apply
+        // ad placement rules across paginated loads.
+        processedContentItemCount: state.feedItems.whereType<Headline>().length,
       );
 
       emit(

lib/headlines-search/bloc/headlines_search_bloc.dart

@@ -112,7 +112,11 @@ class HeadlinesSearchBloc
                 feedItems: headlines,
                 user: currentUser,
                 adConfig: appConfig.adConfig,
-                processedContentItemCount: successState.items.length,
+                // Calculate the count of actual content items (headlines) already in the
+                // feed. This is crucial for the FeedDecoratorService to correctly apply
+                // ad placement rules across paginated loads.
+                processedContentItemCount:
+                    successState.items.whereType<Headline>().length,
               );
               emit(
                 successState.copyWith(