feat(ads): implement AdCacheService for native ad caching

- Create a singleton AdCacheService to cache native ads
- Add methods to get, set, and clear cached ads
- Implement logging for ad cache operations
- Add a method to print the current state of the ad cache
- Ensure proper disposal of AdMob native ad resources when clearing the cache

Author: fulleni

lib/ads/ad_cache_service.dart

@@ -0,0 +1,93 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/ads/models/native_ad.dart';
+import 'package:logging/logging.dart';
+
+/// {@template ad_cache_service}
+/// A singleton service for caching loaded native ad objects.
+///
+/// This service helps to prevent unnecessary re-loading of native ads
+/// when they scroll out of view and then back into view in a scrollable list.
+/// It stores [NativeAd] objects by a unique ID (typically the [AdPlaceholder.id])
+/// and allows for retrieval and clearing of cached ads.
+///
+/// Native ad objects (like `google_mobile_ads.NativeAd`) are stateful and
+/// resource-intensive. Caching them allows for smoother scrolling and
+/// reduces network requests, while still ensuring proper disposal when
+/// the cache is cleared (e.g., on a full feed refresh).
+/// {@endtemplate}
+class AdCacheService {
+  /// Private constructor for the singleton pattern.
+  AdCacheService._internal() : _logger = Logger('AdCacheService');
+
+  /// The single instance of [AdCacheService].
+  static final AdCacheService _instance = AdCacheService._internal();
+
+  /// Factory constructor to provide the singleton instance.
+  factory AdCacheService() => _instance;
+
+  final Logger _logger;
+
+  /// A map to store loaded native ad objects, keyed by their unique ID.
+  ///
+  /// The value is nullable to allow for explicit removal of an ad from the cache.
+  final Map<String, NativeAd?> _cache = {};
+
+  /// Retrieves a [NativeAd] from the cache using its [id].
+  ///
+  /// Returns the cached [NativeAd] if found, otherwise `null`.
+  NativeAd? getAd(String id) {
+    final ad = _cache[id];
+    if (ad != null) {
+      _logger.info('Retrieved ad with ID "$id" from cache.');
+    } else {
+      _logger.info('Ad with ID "$id" not found in cache.');
+    }
+    return ad;
+  }
+
+  /// Adds or updates a [NativeAd] in the cache.
+  ///
+  /// If [ad] is `null`, it effectively removes the entry for [id].
+  void setAd(String id, NativeAd? ad) {
+    if (ad != null) {
+      _cache[id] = ad;
+      _logger.info('Cached ad with ID "$id".');
+    } else {
+      _cache.remove(id);
+      _logger.info('Removed ad with ID "$id" from cache.');
+    }
+  }
+
+  /// Clears all cached native ad objects and disposes their native resources.
+  ///
+  /// This method should be called when the feed is fully refreshed or
+  /// when the application is closing to ensure all native ad resources
+  /// are released.
+  void clearAllAds() {
+    _logger.info('Clearing all cached ads and disposing native resources.');
+    for (final ad in _cache.values) {
+      // Only dispose if the ad is an AdMob native ad.
+      // Placeholder ads do not have native resources to dispose.
+      if (ad?.provider == AdProviderType.admob) {
+        // Cast to the specific AdMob NativeAd type to call dispose.
+        // This is safe because we check the provider type.
+        (ad!.adObject as dynamic).dispose();
+      }
+    }
+    _cache.clear();
+    _logger.info('All cached ads cleared.');
+  }
+
+  /// For debugging: prints the current state of the cache.
+  @visibleForTesting
+  void printCacheState() {
+    _logger.info('Current Ad Cache State:');
+    if (_cache.isEmpty) {
+      _logger.info('  Cache is empty.');
+    } else {
+      _cache.forEach((id, ad) {
+        _logger.info('  ID: $id, Provider: ${ad?.provider}, Template: ${ad?.templateType}');
+      });
+    }
+  }
+}