refactor(headlines-search): improve ad experience in country item widget

- Make the tap action asynchronous to await for the ad to be shown and dismissed
- Add a check to ensure the widget is still in the tree before navigating
- Proceed with navigation after the ad is closed to provide a smoother user experience

Author: fulleni

lib/headlines-search/widgets/country_item_widget.dart

@@ -21,9 +21,15 @@ class CountryItemWidget extends StatelessWidget {
       subtitle: country.isoCode.isNotEmpty
           ? Text(country.isoCode, maxLines: 1, overflow: TextOverflow.ellipsis)
           : null,
-      onTap: () {
-        context.read<InterstitialAdManager>().onPotentialAdTrigger();
-        context.pushNamed(
+      onTap: () async {
+        // Await for the ad to be shown and dismissed.
+        await context.read<InterstitialAdManager>().onPotentialAdTrigger();
+
+        // Check if the widget is still in the tree before navigating.
+        if (!context.mounted) return;
+
+        // Proceed with navigation after the ad is closed.
+        await context.pushNamed(
           Routes.entityDetailsName,
           pathParameters: {'type': ContentType.country.name, 'id': country.id},
         );