_SearchBarState should dispose FocusNode, if it created it. (#134076)

Relanding of revert: flutter/flutter#134072

Verified failed tests succeeded now:

Author: polina-c

packages/flutter/lib/src/material/search_anchor.dart

@@ -1179,6 +1179,9 @@ class _SearchBarState extends State<SearchBar> {
   @override
   void dispose() {
     _internalStatesController.dispose();
+    if (widget.focusNode == null) {
+      _focusNode.dispose();
+    }
     super.dispose();
   }
 

packages/flutter/test/material/search_anchor_test.dart

@@ -6,9 +6,10 @@ import 'dart:ui';
 
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:leak_tracker_flutter_testing/leak_tracker_flutter_testing.dart';
 
 void main() {
-  testWidgets('SearchBar defaults', (WidgetTester tester) async {
+  testWidgetsWithLeakTracking('SearchBar defaults', (WidgetTester tester) async {
     final ThemeData theme = ThemeData(useMaterial3: true);
     final ColorScheme colorScheme = theme.colorScheme;