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

- Add async/await to onTap callback for smoother ad display
- Check if context is still mounted before navigation
- Improve user experience by ensuring ads are fully viewed before page transition

Author: fulleni

lib/headlines-search/widgets/source_item_widget.dart

@@ -22,9 +22,15 @@ class SourceItemWidget extends StatelessWidget {
               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.source.name, 'id': source.id},
         );