Enable explicitChildNodes for the AlertDialog content (#149130)
fixes [AlertDialog content semantics merged](https://github.com/flutter/flutter/issues/147574)
### Code sample
<details>
<summary>expand to view the code sample</summary>
```dart
import 'package:flutter/material.dart';
void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({super.key});
@override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
// showSemanticsDebugger: true,
home: Scaffold(
body: SafeArea(
child: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: <Widget>[
const Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text('Some text'),
Text('More text'),
],
),
Builder(builder: (BuildContext context) {
return ElevatedButton(
onPressed: () {
showDialog<void>(
context: context,
builder: (BuildContext context) {
return AlertDialog(
title: const Text('Dialog Title'),
content: const Column(
children: <Widget>[
Text('Some text'),
Text('More text'),
],
),
actions: <Widget>[
TextButton(
onPressed: () {
Navigator.of(context).pop();
},
child: const Text('Close'),
),
],
);
},
);
},
child: const Text('Open Dialog'),
);
}),
],
),
),
),
),
);
}
}
```
</details>
### Before vs After
6a0e0bfdc5