refactor(ad_platform_type_l10n): add unimplemented error for demo platform type

- Throw UnimplementedError when AdPlatformType.demo is used
- This change clarifies that demo platform type is not intended for dashboard usage
- Emphasizes that demo platform type is for mobile client and demo environment specific use cases

Author: fulleni

lib/shared/extensions/ad_platform_type_l10n.dart

@@ -12,6 +12,10 @@ extension AdPlatformTypeL10n on AdPlatformType {
         return l10n.adPlatformTypeAdmob;
       case AdPlatformType.local:
         return l10n.adPlatformTypeLocal;
+      case AdPlatformType.demo:
+        throw UnimplementedError(
+          'Demo ad platform type is not intended for dashboard usage, rather for mobile client, demo env specific usecase.',
+        );
     }
   }
 }