fix(remote_configs): add missing demo AdPlatformType entries

- Added AdPlatformIdentifiers for AdPlatformType.demo with placeholder values
- Ensures remote_configs.dart fixture now contains all necessary entries

Author: fulleni

lib/src/fixtures/remote_configs.dart

@@ -77,6 +77,11 @@ final remoteConfigsFixturesData = <RemoteConfig>[
             bannerAdId: 'ca-app-pub-3940256099942544/6300978111',
             interstitialAdId: 'ca-app-pub-3940256099942544/1033173712',
           ),
+          AdPlatformType.demo: AdPlatformIdentifiers(
+            nativeAdId: '_',
+            bannerAdId: '_',
+            interstitialAdId: '_',
+          ),
         },
         feedAdConfiguration: FeedAdConfiguration(
           enabled: true,