feat(ads): enhance demo banner ad widget with banner ad shape

- Add bannerAdShape parameter to DemoBannerAdWidget
- Implement logic to determine ad height based on bannerAdShape
- Update ad height calculation to use switch expression
- Add support for square and rectangle banner ad shapes

Author: fulleni

lib/ads/widgets/demo_banner_ad_widget.dart

@@ -11,19 +11,26 @@ import 'package:ui_kit/ui_kit.dart';
 /// {@endtemplate}
 class DemoBannerAdWidget extends StatelessWidget {
   /// {@macro demo_banner_ad_widget}
-  const DemoBannerAdWidget({this.headlineImageStyle, super.key});
+  const DemoBannerAdWidget({this.headlineImageStyle, this.bannerAdShape, super.key});
 
   /// The user's preference for feed layout, used to determine the ad's visual size.
   final HeadlineImageStyle? headlineImageStyle;
 
+  /// The preferred shape for banner ads, used for in-article banners.
+  final BannerAdShape? bannerAdShape;
+
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
 
-    // Determine the height based on the headlineImageStyle, mimicking real ad widgets.
-    final adHeight = headlineImageStyle == HeadlineImageStyle.largeThumbnail
-        ? 250 // Height for mediumRectangle banner
-        : 50; // Height for standard banner
+    // Determine the height based on the bannerAdShape if provided.
+    // If bannerAdShape is square, use height for mediumRectangle (250).
+    // Otherwise, use height for standard banner (50).
+    final adHeight = switch (bannerAdShape) {
+      BannerAdShape.square => 250,
+      BannerAdShape.rectangle => 50,
+      _ => 50, // Default to standard banner height if shape is null or unknown
+    };
 
     return Card(
       margin: const EdgeInsets.symmetric(