Fix NavigationBar example overflow alignment (#159034)

Fix the NavigationBar example overflow alignment.

### Before: 
<img
src="https://github.com/user-attachments/assets/a6cf2cbb-ac32-482c-9aed-8cac15152a0d"
height="450" />


### After:
<img
src="https://github.com/user-attachments/assets/d476c617-edd3-449d-93cf-4a9a87c26d98"
height="450" />



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examples/api/lib/material/navigation_bar/navigation_bar.1.dart

@@ -63,6 +63,8 @@ class _NavigationExampleState extends State<NavigationExample> {
             const SizedBox(height: 10),
             OverflowBar(
               spacing: 10.0,
+              overflowAlignment: OverflowBarAlignment.center,
+              overflowSpacing: 10.0,
               children: <Widget>[
                 ElevatedButton(
                   onPressed: () {

examples/api/test/material/navigation_bar/navigation_bar.1_test.dart

@@ -38,6 +38,17 @@ void main() {
 
     expect(find.text('Label behavior: alwaysHide'), findsOneWidget);
     navigationBarWidget = tester.firstWidget(find.byType(NavigationBar));
-    expect(navigationBarWidget.labelBehavior, NavigationDestinationLabelBehavior.alwaysHide);
+    expect(
+      navigationBarWidget.labelBehavior,
+      NavigationDestinationLabelBehavior.alwaysHide,
+    );
+  });
+
+  testWidgets('Overflow buttons are aligned in the center', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.NavigationBarApp());
+
+    final OverflowBar overflowBar = tester.widget<OverflowBar>(find.byType(OverflowBar));
+    expect(overflowBar.overflowAlignment, OverflowBarAlignment.center);
+    expect(overflowBar.overflowSpacing, 10.0);
   });
 }