fix(headlines-search): inject ad placeholders instead of ads in search results and pagination

- Replace `injectAds` with `injectAdPlaceholders` in search pagination and results
- Clear previous ad cache for new searches
- Update comments to reflect change from ads to ad placeholders

Author: fulleni

lib/headlines-search/bloc/headlines_search_bloc.dart

@@ -5,6 +5,7 @@ import 'package:bloc_concurrency/bloc_concurrency.dart';
 import 'package:core/core.dart';
 import 'package:data_repository/data_repository.dart';
 import 'package:equatable/equatable.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/ads/ad_cache_service.dart';
 import 'package:flutter_news_app_mobile_client_full_source_code/ads/models/ad_theme_style.dart';
 import 'package:flutter_news_app_mobile_client_full_source_code/app/bloc/app_bloc.dart';
 import 'package:flutter_news_app_mobile_client_full_source_code/shared/services/feed_decorator_service.dart';
@@ -108,8 +109,9 @@ class HeadlinesSearchBloc
                 );
                 return;
               }
-              // For search pagination, only inject ads.
-              final injectedItems = await _feedDecoratorService.injectAds(
+              // For search pagination, only inject ad placeholders.
+              final injectedItems =
+                  await _feedDecoratorService.injectAdPlaceholders(
                 feedItems: headlines,
                 user: currentUser,
                 adConfig: appConfig.adConfig,
@@ -185,7 +187,8 @@ class HeadlinesSearchBloc
       }
     }
 
-    // New search
+    // New search, clear previous ad cache.
+    AdCacheService().clearAllAds();
     emit(
       HeadlinesSearchLoading(
         lastSearchTerm: searchTerm,
@@ -216,8 +219,8 @@ class HeadlinesSearchBloc
             );
             return;
           }
-          // For search results, only inject ads.
-          processedItems = await _feedDecoratorService.injectAds(
+          // For search results, only inject ad placeholders.
+          processedItems = await _feedDecoratorService.injectAdPlaceholders(
             feedItems: headlines,
             user: currentUser,
             adConfig: appConfig.adConfig,