feat: add NewsItem and BriefingFeed models with tests

lib/data/models/briefing_feed.dart

@@ -0,0 +1,20 @@
+class BriefingFeed {
+  final int id;
+  final String title;
+  final String url;
+  final String? category;
+
+  const BriefingFeed({
+    required this.id,
+    required this.title,
+    required this.url,
+    this.category,
+  });
+
+  factory BriefingFeed.fromJson(Map<String, dynamic> json) => BriefingFeed(
+        id: json['id'] as int,
+        title: json['title'] as String? ?? '',
+        url: json['url'] as String? ?? '',
+        category: json['category'] as String?,
+      );
+}