message list: Add search button to app bar and related tests.

Adds a search icon button in the message list app bar
that appears on the combined feed narrow. Uses the same localized 'Search' text
for consistency with the message list search button.

Also adds a focused test in the existing 'app bar' group inside message_list_test that verifies
the search button appears on combined feed and navigates correctly to
the search page when tapped.

Author: Komal Singh

lib/widgets/message_list.dart

@@ -416,6 +416,12 @@ abstract class _MessageListAppBar {
     List<Widget> actions = [];
     switch (narrow) {
       case CombinedFeedNarrow():
+        actions.add(IconButton(
+          icon: const Icon(ZulipIcons.search),
+          tooltip: zulipLocalizations.searchMessagesPageTitle,
+          onPressed: () => Navigator.push(context,
+            MessageListPage.buildRoute(context: context,
+              narrow: KeywordSearchNarrow('')))));
       case MentionsNarrow():
       case StarredMessagesNarrow():
       case KeywordSearchNarrow():

test/widgets/message_list_test.dart

@@ -363,6 +363,42 @@ void main() {
 
       check(find.text('DMs with Muted user, User 2, Muted user')).findsOne();
     });
+
+    testWidgets('search button on combined feed navigates to search page', (tester) async {
+      // Set up navigation tracking
+      final pushedRoutes = <Route<dynamic>>[];
+      final testNavObserver = TestNavigatorObserver()
+        ..onPushed = (route, prevRoute) => pushedRoutes.add(route);
+
+      // Set up the combined feed view with navigation observer
+      await setupMessageListPage(tester,
+        narrow: const CombinedFeedNarrow(),
+        messages: [],
+        navObservers: [testNavObserver]);
+
+      // Verify that the search button is present in the app bar
+      final searchButtonFinder = find.descendant(
+        of: find.byType(ZulipAppBar),
+        matching: find.byIcon(ZulipIcons.search));
+      check(searchButtonFinder).findsOne();
+
+      // Clear any initial navigation that happened during setup
+      pushedRoutes.clear();
+
+      // Mock the API call for the search page
+      connection.prepare(json: eg.newestGetMessagesResult(
+        foundOldest: true, messages: []).toJson());
+
+      // Tap the search button in the app bar
+      await tester.tap(searchButtonFinder);
+      await tester.pump();
+
+      // Verify that we navigated to the search page with an empty search query
+      check(pushedRoutes).single.isA<WidgetRoute>().page
+        .isA<MessageListPage>()
+        .initNarrow.equals(KeywordSearchNarrow(''));
+      await tester.pump(Duration.zero); // Allow message list fetch to complete
+    });
   });
 
   group('no-messages placeholder', () {