fixes [`SearchAnchor` triggers extra search operations](https://github.com/flutter/flutter/issues/139880)

### Code sample

<details>
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

Future<List<String>> createFuture() async {
  return List.generate(1000, (index) => "Hello World!");
}

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Flutter Demo',
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final SearchController controller = SearchController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            SearchAnchor(
              searchController: controller,
              suggestionsBuilder: (suggestionsContext, controller) {
                final resultFuture = createFuture();
                return [
                  FutureBuilder(
                    future: resultFuture,
                    builder: ((context, snapshot) {
                      if (snapshot.connectionState != ConnectionState.done) {
                        return const LinearProgressIndicator();
                      }
                      final result = snapshot.data;
                      if (result == null) {
                        return const LinearProgressIndicator();
                      }
                      return ListView.builder(
                        shrinkWrap: true,
                        physics: const NeverScrollableScrollPhysics(),
                        itemCount: result.length,
                        itemBuilder: (BuildContext context, int index) {
                          final root = result[index];
                          return ListTile(
                            leading: const Icon(Icons.article),
                            title: Text(root),
                            subtitle: Text(
                              root,
                              overflow: TextOverflow.ellipsis,
                              style: TextStyle(
                                color: Theme.of(suggestionsContext)
                                    .colorScheme
                                    .onSurfaceVariant,
                              ),
                            ),
                            onTap: () {},
                          );
                        },
                      );
                    }),
                  ),
                ];
              },
              builder: (BuildContext context, SearchController controller) {
                return IconButton(
                  onPressed: () {
                    controller.openView();
                  },
                  icon: const Icon(Icons.search),
                );
              },
            ),
          ],
        ),
      ),
    );
  }
}

```

</details>

### Before

https://github.com/flutter/flutter/assets/48603081/69f6dfdc-9f92-4d2e-8a3e-984fce25f9e4

### After

https://github.com/flutter/flutter/assets/48603081/be105e2c-51d8-4cb0-a75b-f5f41d948e5e