fix(feed): resolve type error on returning from multi-select page

Fixes a `_TypeError` that occurred when popping the `MultiSelectSearchPage`. The error was caused by a type mismatch between the `Set<dynamic>` returned by the generic page and the `Future<Set<Country>>` expected by the calling page.

The solution is to change the expected return type of `pushNamed` to `Set<dynamic>` and then safely cast the resulting set to `Set<Country>` before updating the state. This ensures type safety and resolves the runtime crash.

Author: fulleni

lib/headlines-feed/view/source_list_filter_page.dart

@@ -95,7 +95,7 @@ class _SourceListFilterPageState extends State<SourceListFilterPage> {
             ),
             trailing: const Icon(Icons.chevron_right),
             onTap: () async {
-              final result = await context.pushNamed<Set<Country>>(
+              final result = await context.pushNamed<Set<dynamic>>(
                 Routes.multiSelectSearchName,
                 extra: {
                   'title': l10n.headlinesFeedFilterSourceCountryLabel,
@@ -106,7 +106,9 @@ class _SourceListFilterPageState extends State<SourceListFilterPage> {
               );
 
               if (result != null && mounted) {
-                setState(() => _selectedHeadquarterCountries = result);
+                setState(
+                  () => _selectedHeadquarterCountries = result.cast<Country>(),
+                );
               }
             },
           ),