feat(ads): introduce AdPlaceholder model

- Create a new StatelessWidget class AdPlaceholder
- Implement EquatableMixin for state comparison
- Add documentation explaining the purpose and usage of AdPlaceholder
- Define a unique ID for each ad placeholder instance
- Include this model in the ads library for feed item ad placeholders

Author: fulleni

lib/ads/models/ad_placeholder.dart

@@ -0,0 +1,24 @@
+import 'package:core/core.dart';
+import 'package:equatable/equatable.dart';
+
+/// {@template ad_placeholder}
+/// A stateless [FeedItem] that acts as a marker for an ad slot in the feed.
+///
+/// This model is used in the BLoC's state to indicate where an ad should be
+/// displayed, without holding the actual, stateful native ad object.
+/// The actual ad loading and display logic is handled by a dedicated widget
+/// that interacts with an ad cache.
+/// {@endtemplate}
+class AdPlaceholder extends FeedItem with EquatableMixin {
+  /// {@macro ad_placeholder}
+  const AdPlaceholder({required this.id}) : super(type: 'ad_placeholder');
+
+  /// A unique identifier for this specific ad placeholder instance.
+  ///
+  /// This ID is used by the ad loading widget to request a specific ad
+  /// from the ad cache or to load a new one if not found.
+  final String id;
+
+  @override
+  List<Object?> get props => [id, type];
+}