feat(search): add SearchModelType enum

- Defines searchable model types
- Includes display names
- Includes toJson method

Author: fulleni

lib/headlines-search/models/search_model_type.dart

@@ -0,0 +1,28 @@
+/// Defines the types of models that can be searched.
+enum SearchModelType {
+  headline,
+  category,
+  country,
+  source;
+
+  /// Returns a user-friendly display name for the enum value.
+  ///
+  /// This should ideally be localized using context.l10n,
+  /// but for simplicity in this step, we'll use direct strings.
+  /// TODO(Cline): Localize these display names.
+  String get displayName {
+    switch (this) {
+      case SearchModelType.headline:
+        return 'Headlines';
+      case SearchModelType.category:
+        return 'Categories';
+      case SearchModelType.country:
+        return 'Countries';
+      case SearchModelType.source:
+        return 'Sources';
+    }
+  }
+
+  /// Returns the string representation for API query parameters.
+  String toJson() => name;
+}