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" />



*List which issues are fixed by this PR. You must list at least one
issue. An issue is not required if the PR fixes something trivial like a
typo.*

*If you had to change anything in the [flutter/tests] repo, include a
link to the migration guide as per the [breaking change policy].*

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: yaostyle

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);
   });
 }