feat(ads): implement AdNavigatorObserver for interstitial ads

- Add AdNavigatorObserver class to listen to route changes and trigger interstitial ad display
- Integrate with AppBloc for remote config and user ad frequency settings
- Implement logic to show interstitial ads based on navigation events
- Include logging for ad-related actions

Author: fulleni

lib/ads/ad_navigator_observer.dart

@@ -0,0 +1,132 @@
+import 'dart:async';
+
+import 'package:core/core.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_bloc/flutter_bloc.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/ads/ad_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:logging/logging.dart';
+
+/// {@template ad_navigator_observer}
+/// A [NavigatorObserver] that listens to route changes and triggers
+/// interstitial ad display based on [RemoteConfig] settings.
+///
+/// This observer is responsible for:
+/// 1. Tracking page transitions to determine when an interstitial ad should be shown.
+/// 2. Requesting an interstitial ad from the [AdService] when the criteria are met.
+/// 3. Showing the interstitial ad to the user.
+///
+/// It interacts with the [AppBloc] to get the current [RemoteConfig] and
+/// user's ad frequency settings, and to dispatch events for page transitions.
+/// {@endtemplate}
+class AdNavigatorObserver extends NavigatorObserver {
+  /// {@macro ad_navigator_observer}
+  AdNavigatorObserver({
+    required this.appBloc,
+    required this.adService,
+    Logger? logger,
+  }) : _logger = logger ?? Logger('AdNavigatorObserver');
+
+  final AppBloc appBloc;
+  final AdService adService;
+  final Logger _logger;
+
+  /// Tracks the number of page transitions since the last interstitial ad was shown.
+  /// This is managed by the AppBloc.
+
+  @override
+  void didPush(Route<dynamic> route, Route<dynamic>? previousRoute) {
+    super.didPush(route, previousRoute);
+    _logger.info('Route pushed: ${route.settings.name}');
+    _handlePageTransition();
+  }
+
+  @override
+  void didPop(Route<dynamic> route, Route<dynamic>? previousRoute) {
+    super.didPop(route, previousRoute);
+    _logger.info('Route popped: ${route.settings.name}');
+    _handlePageTransition();
+  }
+
+  /// Handles a page transition event.
+  ///
+  /// Dispatches an [AppPageTransitioned] event to the [AppBloc] to update
+  /// the transition count and potentially trigger an interstitial ad.
+  void _handlePageTransition() {
+    // Only consider actual page routes, not overlays or dialogs.
+    // This check is a simplification; a more robust solution might inspect
+    // route types or use GoRouter's specific navigation events.
+    if (navigator?.currentRoutes.last.settings.name != null) {
+      appBloc.add(const AppPageTransitioned());
+    }
+  }
+
+  /// Requests and shows an interstitial ad if conditions are met.
+  ///
+  /// This method is called by the [AppBloc] when it determines an ad is due.
+  Future<void> showInterstitialAd() async {
+    final remoteConfig = appBloc.state.remoteConfig;
+    final user = appBloc.state.user;
+
+    if (remoteConfig == null || !remoteConfig.adConfig.enabled) {
+      _logger.info('Interstitial ads disabled or remote config not available.');
+      return;
+    }
+
+    final adConfig = remoteConfig.adConfig;
+    final interstitialConfig = adConfig.interstitialAdConfiguration;
+
+    if (!interstitialConfig.enabled) {
+      _logger.info('Interstitial ads are specifically disabled in config.');
+      return;
+    }
+
+    // Create AdThemeStyle from current theme for ad loading.
+    // This requires a BuildContext, which is not directly available here.
+    // The AppBloc will need to provide the AdThemeStyle or the AdService
+    // will need to be able to create it. For now, we'll assume a default
+    // or that the AdService can handle it.
+    //
+    // TODO(fulleni): Revisit how AdThemeStyle is passed for interstitial ads.
+    // For now, a placeholder AdThemeStyle is used.
+    final adThemeStyle = AdThemeStyle.fromTheme(ThemeData());
+
+    _logger.info('Attempting to load interstitial ad...');
+    final interstitialAd = await adService.getInterstitialAd(
+      adConfig: adConfig,
+      adThemeStyle: adThemeStyle,
+    );
+
+    if (interstitialAd != null) {
+      _logger.info('Interstitial ad loaded. Showing...');
+      // Show the AdMob interstitial ad.
+      if (interstitialAd.provider == AdPlatformType.admob &&
+          interstitialAd.adObject is admob.InterstitialAd) {
+        final admobInterstitialAd =
+            interstitialAd.adObject as admob.InterstitialAd;
+        admobInterstitialAd.fullScreenContentCallback =
+            admob.FullScreenContentCallback(
+          onAdDismissedFullScreenContent: (ad) {
+            _logger.info('Interstitial Ad dismissed.');
+            ad.dispose();
+          },
+          onAdFailedToShowFullScreenContent: (ad, error) {
+            _logger.severe('Interstitial Ad failed to show: $error');
+            ad.dispose();
+          },
+          onAdShowedFullScreenContent: (ad) {
+            _logger.info('Interstitial Ad showed.');
+          },
+        );
+        await admobInterstitialAd.show();
+      } else if (interstitialAd.provider == AdPlatformType.local &&
+          interstitialAd.adObject is LocalInterstitialAd) {
+        // TODO(fulleni): Implement showing local interstitial ad (e.g., via a dialog).
+        _logger.info('Showing local interstitial ad (placeholder).');
+      }
+    } else {
+      _logger.info('No interstitial ad loaded.');
+    }
+  }
+}