fixes [Invisible SliverAppBar title in Material 3 light theme](https://github.com/flutter/flutter/issues/138296) 
fixes [`FlexibleSpaceBar` title is misaligned without the leading widget](https://github.com/flutter/flutter/issues/138608)

Previous attempt https://github.com/flutter/flutter/pull/138611

--- 

### Description

 - fixes the `FlexibleSpaceBar` centered title position when there is a leading widget.
 - fixes the `FlexibleSpaceBar` title color for Material 3.
 - Added documentation when using a long `FlexibleSpaceBar` title and update its test.
 - Improved documentation of default title padding.

### Code sample

### 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 const MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Example(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return const Scaffold(
      body: SafeArea(
          child: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            leading: Icon(Icons.favorite_rounded),
            flexibleSpace: FlexibleSpaceBar(
              title: ColoredBox(
                color: Color(0xffff0000),
                child: Text('SliverAppBar'),
              ),
            ),
          ),
        ],
      )),
    );
  }
}
```

</details>

###  Before

![Screenshot 2024-01-03 at 18 02 25](https://github.com/flutter/flutter/assets/48603081/92ae1062-c78f-4005-8e28-85af617acd60)

### After

![Screenshot 2024-01-03 at 18 02 16](https://github.com/flutter/flutter/assets/48603081/2ef97108-9b50-44f7-a303-018ff1b28db6)