feat(data_client): add CountryInMemoryClient for enhanced country filtering

- Implement CountryInMemoryClient to filter countries based on active sources and headlines
- Extend DataClient<Country> with custom logic for hasActiveSources and hasActiveHeadlines filters
- Use static fixture data for filtering operations
- Delegate standard DataClient methods to a wrapped DataClient<Country> instance

Author: fulleni

lib/shared/data/clients/country_inmemory_client.dart

@@ -0,0 +1,134 @@
+import 'package:core/core.dart';
+import 'package:data_client/data_client.dart';
+
+/// {@template country_inmemory_client}
+/// A specialized in-memory data client for [Country] models that extends
+/// the functionality of a generic [DataClient<Country>] by adding custom
+/// filtering logic for `hasActiveSources` and `hasActiveHeadlines` filters.
+///
+/// This client acts as a decorator, wrapping an existing [DataClient<Country>]
+/// (typically [DataInMemory<Country>]) and intercepting `readAll` calls
+/// to apply application-specific filtering using static fixture data.
+/// All other [DataClient] methods are delegated directly to the wrapped client.
+/// {@endtemplate}
+class CountryInMemoryClient implements DataClient<Country> {
+  /// {@macro country_inmemory_client}
+  ///
+  /// Requires a [decoratedClient] to which standard data operations will be
+  /// delegated. Also requires [allSources] and [allHeadlines] (typically
+  /// static fixture data from the `core` package) to perform custom filtering.
+  const CountryInMemoryClient({
+    required DataClient<Country> decoratedClient,
+    required List<Source> allSources,
+    required List<Headline> allHeadlines,
+  })  : _decoratedClient = decoratedClient,
+        _allSources = allSources,
+        _allHeadlines = allHeadlines;
+
+  final DataClient<Country> _decoratedClient;
+  final List<Source> _allSources;
+  final List<Headline> _allHeadlines;
+
+  @override
+  Future<SuccessApiResponse<List<Map<String, dynamic>>>> aggregate({
+    required List<Map<String, dynamic>> pipeline,
+    String? userId,
+  }) {
+    return _decoratedClient.aggregate(pipeline: pipeline, userId: userId);
+  }
+
+  @override
+  Future<SuccessApiResponse<int>> count({
+    String? userId,
+    Map<String, dynamic>? filter,
+  }) {
+    return _decoratedClient.count(userId: userId, filter: filter);
+  }
+
+  @override
+  Future<SuccessApiResponse<Country>> create({
+    required Country item,
+    String? userId,
+  }) {
+    return _decoratedClient.create(item: item, userId: userId);
+  }
+
+  @override
+  Future<void> delete({required String id, String? userId}) {
+    return _decoratedClient.delete(id: id, userId: userId);
+  }
+
+  @override
+  Future<SuccessApiResponse<Country>> read({
+    required String id,
+    String? userId,
+  }) {
+    return _decoratedClient.read(id: id, userId: userId);
+  }
+
+  @override
+  Future<SuccessApiResponse<PaginatedResponse<Country>>> readAll({
+    String? userId,
+    Map<String, dynamic>? filter,
+    PaginationOptions? pagination,
+    List<SortOption>? sort,
+  }) async {
+    // First, get the initial list of countries from the decorated client.
+    // This handles generic filters, sorting, and pagination.
+    final response = await _decoratedClient.readAll(
+      userId: userId,
+      filter: filter,
+      pagination: pagination,
+      sort: sort,
+    );
+
+    var filteredCountries = response.data.items;
+
+    // Apply custom filters if present
+    final hasActiveSources = filter?['hasActiveSources'] == true;
+    final hasActiveHeadlines = filter?['hasActiveHeadlines'] == true;
+
+    if (hasActiveSources) {
+      final countriesWithActiveSources = _allSources
+          .where((source) => source.status == ContentStatus.active)
+          .map((source) => source.headquarters.id)
+          .toSet();
+
+      filteredCountries = filteredCountries
+          .where(
+            (country) => countriesWithActiveSources.contains(country.id),
+          )
+          .toList();
+    }
+
+    if (hasActiveHeadlines) {
+      final countriesWithActiveHeadlines = _allHeadlines
+          .where((headline) => headline.status == ContentStatus.active)
+          .map((headline) => headline.eventCountry.id)
+          .toSet();
+
+      filteredCountries = filteredCountries
+          .where(
+            (country) => countriesWithActiveHeadlines.contains(country.id),
+          )
+          .toList();
+    }
+
+    // Return a new PaginatedResponse with the potentially further filtered items.
+    // The cursor and hasMore logic from the original response are preserved,
+    // but the items list is updated.
+    return SuccessApiResponse(
+      data: response.data.copyWith(items: filteredCountries),
+      metadata: response.metadata,
+    );
+  }
+
+  @override
+  Future<SuccessApiResponse<Country>> update({
+    required String id,
+    required Country item,
+    String? userId,
+  }) {
+    return _decoratedClient.update(id: id, item: item, userId: userId);
+  }
+}