feat(ads): add interstitial ad trigger on source item tap

- Import InterstitialAdManager in source_item_widget.dart
- Add context read for InterstitialAdManager on tap event
- Call onPotentialAdTrigger method when source item is tapped

Author: fulleni

lib/headlines-search/widgets/source_item_widget.dart

@@ -1,5 +1,7 @@
 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/interstitial_ad_manager.dart';
 import 'package:flutter_news_app_mobile_client_full_source_code/router/routes.dart';
 import 'package:go_router/go_router.dart';
 
@@ -21,6 +23,9 @@ class SourceItemWidget extends StatelessWidget {
             )
           : null,
       onTap: () {
+        context
+            .read<InterstitialAdManager>()
+            .onPotentialAdTrigger(context: context);
         context.pushNamed(
           Routes.entityDetailsName,
           pathParameters: {'type': ContentType.source.name, 'id': source.id},