fix(ads): adjust demo native ad height

- Decrease the height for the large thumbnail template from 340 to 250
- This change ensures a more appropriate and consistent ad appearance

Author: fulleni

lib/ads/widgets/demo_native_ad_widget.dart

@@ -23,7 +23,7 @@ class DemoNativeAdWidget extends StatelessWidget {
 
     // Determine the height based on the headlineImageStyle, mimicking real ad widgets.
     final adHeight = headlineImageStyle == HeadlineImageStyle.largeThumbnail
-        ? 340 // Height for medium native ad template
+        ? 250 // Height for medium native ad template
         : 120; // Height for small native ad template
 
     return Card(