refactor(headlines_feed): remove dead code from feed page

Removes the unreachable `if` branches for `CallToActionItem` and `ContentCollectionItem` from the `itemBuilder` in `HeadlinesFeedPage`.

The rendering of these decorators is now fully encapsulated within the `FeedDecoratorLoaderWidget`, which is built when a `DecoratorPlaceholder` is encountered. This change cleans up the rendering logic and removes dead code. A `ValueKey` is also added to the `FeedDecoratorLoaderWidget` to ensure its state is managed correctly within the list.

Author: fulleni

lib/headlines-feed/view/headlines_feed_page.dart

@@ -315,15 +315,16 @@ class _HeadlinesFeedPageState extends State<HeadlinesFeedPage>
                               adThemeStyle: AdThemeStyle.fromTheme(theme),
                               adConfig: adConfig,
                             );
-                          } else if (item is CallToActionItem) {
-                            // This case is now handled by FeedDecoratorLoaderWidget.
-                            return const SizedBox.shrink();
-                          } else if (item is ContentCollectionItem) {
-                            // This case is now handled by FeedDecoratorLoaderWidget.
-                            return const SizedBox.shrink();
                           } else if (item is DecoratorPlaceholder) {
-                            return const FeedDecoratorLoaderWidget();
+                            // The FeedDecoratorLoaderWidget is responsible for
+                            // determining which non-ad decorator to show and
+                            // managing its entire lifecycle. A ValueKey is
+                            // used to ensure its state is preserved correctly.
+                            return FeedDecoratorLoaderWidget(
+                              key: ValueKey(item.id),
+                            );
                           }
+                          // Return an empty box for any other unhandled item types.
                           return const SizedBox.shrink();
                         },
                       ),