feat(ads): implement NoOpAdProvider for unsupported platforms

- Add NoOpAdProvider class implementing AdProvider interface
- Provide a "no-operation" implementation for platforms without native ad SDKs
- Initialize method logs a message without performing any actions
- loadNativeAd method returns a placeholder NativeAd for UI consistency

Author: fulleni

lib/ads/no_op_ad_provider.dart

@@ -0,0 +1,57 @@
+import 'package:core/core.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/ads/ad_provider.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/ads/models/ad_theme_style.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/ads/models/native_ad.dart';
+import 'package:logging/logging.dart';
+import 'package:uuid/uuid.dart';
+
+/// {@template no_op_ad_provider}
+/// A "no-operation" implementation of [AdProvider] for platforms where
+/// native ad SDKs are not supported (e.g., web).
+///
+/// This provider's `initialize` method does nothing, and its `loadNativeAd`
+/// method returns a [NativeAd] with [AdProviderType.placeholder]. This ensures
+/// that the application's UI can still render an "ad slot" for visual
+/// consistency in demo environments, without attempting to load actual native
+/// ads that would cause platform exceptions.
+/// {@endtemplate}
+class NoOpAdProvider implements AdProvider {
+  /// {@macro no_op_ad_provider}
+  NoOpAdProvider({Logger? logger})
+      : _logger = logger ?? Logger('NoOpAdProvider');
+
+  final Logger _logger;
+  final Uuid _uuid = const Uuid();
+
+  /// This method does nothing as there's no SDK to initialize.
+  ///
+  /// It logs a message to indicate its no-op nature.
+  @override
+  Future<void> initialize() async {
+    _logger.info('No-Op Ad Provider initialized (no actual SDK to init).');
+  }
+
+  /// Loads a placeholder native ad.
+  ///
+  /// This method does not interact with any external ad network. Instead, it
+  /// creates a [NativeAd] object with [AdProviderType.placeholder], which
+  /// signals to the UI that a generic placeholder should be rendered.
+  ///
+  /// The [imageStyle] and [adThemeStyle] parameters are accepted for API
+  /// compatibility but are not used to load a real ad.
+  @override
+  Future<NativeAd?> loadNativeAd({
+    required HeadlineImageStyle imageStyle,
+    required AdThemeStyle adThemeStyle,
+  }) async {
+    _logger.info('Loading placeholder native ad.');
+    // Return a dummy NativeAd object with a placeholder type.
+    // The `adObject` can be any non-null object, as it won't be used by
+    // the placeholder rendering widget.
+    return NativeAd(
+      id: _uuid.v4(),
+      provider: AdProviderType.placeholder,
+      adObject: Object(), // Dummy object
+    );
+  }
+}