feat(headlines-feed): implement HeadlinesFilterState for filter feature

- Define HeadlinesFilterStatus enum for different fetching statuses
- Create HeadlinesFilterState class to hold filter options and selections
- Include properties for topics, sources, countries, and followed items
- Implement copyWith method for state immutability
- Add error handling for failure status

Author: fulleni

lib/headlines-feed/bloc/headlines_filter_state.dart

@@ -0,0 +1,103 @@
+part of 'headlines_filter_bloc.dart';
+
+/// Enum representing the different statuses of the filter data fetching.
+enum HeadlinesFilterStatus {
+  /// Initial state, no data loaded yet.
+  initial,
+
+  /// Currently loading all filter data (topics, sources, countries).
+  loading,
+
+  /// Successfully loaded all filter data.
+  success,
+
+  /// An error occurred while fetching filter data.
+  failure,
+}
+
+/// {@template headlines_filter_state}
+/// Represents the state for the centralized headlines filter feature.
+///
+/// This state holds all available filter options (topics, sources, countries)
+/// and the user's current temporary selections, along with loading/error status.
+/// {@endtemplate}
+final class HeadlinesFilterState extends Equatable {
+  /// {@macro headlines_filter_state}
+  const HeadlinesFilterState({
+    this.status = HeadlinesFilterStatus.initial,
+    this.allTopics = const [],
+    this.allSources = const [],
+    this.allCountries = const [],
+    this.selectedTopics = const {},
+    this.selectedSources = const {},
+    this.selectedCountries = const {},
+    this.isUsingFollowedItems = false,
+    this.error,
+  });
+
+  /// The current status of fetching filter data.
+  final HeadlinesFilterStatus status;
+
+  /// All available [Topic] objects that can be used for filtering.
+  final List<Topic> allTopics;
+
+  /// All available [Source] objects that can be used for filtering.
+  final List<Source> allSources;
+
+  /// All available [Country] objects that can be used for filtering.
+  final List<Country> allCountries;
+
+  /// The set of [Topic] objects currently selected by the user.
+  final Set<Topic> selectedTopics;
+
+  /// The set of [Source] objects currently selected by the user.
+  final Set<Source> selectedSources;
+
+  /// The set of [Country] objects currently selected by the user.
+  final Set<Country> selectedCountries;
+
+  /// Flag indicating if the filter is currently set to "Apply my followed items".
+  final bool isUsingFollowedItems;
+
+  /// An optional error object if the status is [HeadlinesFilterStatus.failure].
+  final HttpException? error;
+
+  /// Creates a copy of this state with the given fields replaced.
+  HeadlinesFilterState copyWith({
+    HeadlinesFilterStatus? status,
+    List<Topic>? allTopics,
+    List<Source>? allSources,
+    List<Country>? allCountries,
+    Set<Topic>? selectedTopics,
+    Set<Source>? selectedSources,
+    Set<Country>? selectedCountries,
+    bool? isUsingFollowedItems,
+    HttpException? error,
+    bool clearError = false,
+  }) {
+    return HeadlinesFilterState(
+      status: status ?? this.status,
+      allTopics: allTopics ?? this.allTopics,
+      allSources: allSources ?? this.allSources,
+      allCountries: allCountries ?? this.allCountries,
+      selectedTopics: selectedTopics ?? this.selectedTopics,
+      selectedSources: selectedSources ?? this.selectedSources,
+      selectedCountries: selectedCountries ?? this.selectedCountries,
+      isUsingFollowedItems: isUsingFollowedItems ?? this.isUsingFollowedItems,
+      error: clearError ? null : error ?? this.error,
+    );
+  }
+
+  @override
+  List<Object?> get props => [
+    status,
+    allTopics,
+    allSources,
+    allCountries,
+    selectedTopics,
+    selectedSources,
+    selectedCountries,
+    isUsingFollowedItems,
+    error,
+  ];
+}