Repository: flutter
Issue: WidgetSpan text cannot be copied using SelectableText.rich
Labels: a: text input,framework,has reproducible steps,P2,team-framework,triaged-framework,found in release: 3.27,found in release: 3.28

### Steps to reproduce

```dart 
SelectableText.rich(
      TextSpan(
        children: [
         WidgetSpan(
          alignment: PlaceholderAlignment.middle,
          child: Container(
              decoration: BoxDecoration(color: Colors.red, borderRadius: BorderRadius.circular(10)),
              padding: const EdgeInsets.only(right: 5.0),
              child: RichText(
                overflow: TextOverflow.visible,
                text: TextSpan(
                  text: '12312 3123',
                ),
              )
           ),
        ),
          TextSpan(
            text: '    $time',
            style: TextStyle(color: Colors.transparent),
          ),
        ],
      ),
    );
```
### Expected results

'12312 3123'

### Actual results
￼

It's possible add placeholder to WidgetSpan with String vale ?

### Code sample

<details></details>

### Screenshots or Video

_No response_

### Logs

_No response_

### Flutter Doctor output

<details open><summary>Doctor output</summary>

```console
[√] Flutter (Channel stable, 3.27.1, on Microsoft Windows [Version 10.0.14393], locale ru-RU)
    • Flutter version 3.27.1 on channel stable at C:\Users\Poloten\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 17025dd882 (2 days ago), 2024-12-17 03:23:09 +0900
    • Engine revision cb4b5fff73
    • Dart version 3.6.0
    • DevTools version 2.40.2

[√] Windows Version (Installed version of Windows is version 10 or higher)

[√] Android toolchain - develop for Android devices (Android SDK version 35.0.0)
    • Android SDK at C:\Users\Poloten\AppData\Local\Android\Sdk
    • Platform android-35, build-tools 35.0.0
    • ANDROID_HOME = C:\Users\Poloten\AppData\Local\Android\Sdk
    • Java binary at: C:\Program Files\Android\Android Studio\jbr\bin\java
    • Java version OpenJDK Runtime Environment (build 17.0.11+0--11852314)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.7.1)
    • Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
    • Visual Studio Community 2022 version 17.7.34009.444
    • Windows 10 SDK version 10.0.22621.0

[√] Android Studio (version 2024.1)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin can be installed from:
       https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
       https://plugins.jetbrains.com/plugin/6351-dart
    • android-studio-dir = C:\Program Files\Android\Android Studio
    • Java version OpenJDK Runtime Environment (build 17.0.11+0--11852314)

[√] VS Code (version 1.96.0)
    • VS Code at C:\Users\Poloten\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.102.0

[√] Connected device (4 available)
    • sdk gphone64 x86 64 (mobile) • emulator-5554 • android-x64    • Android 13 (API 33) (emulator)
    • Windows (desktop)            • windows       • windows-x64    • Microsoft Windows [Version 10.0.14393]
    • Chrome (web)                 • chrome        • web-javascript • Google Chrome 131.0.6778.140
    • Edge (web)                   • edge          • web-javascript • Microsoft Edge 130.0.2849.68

[√] Network resources
    • All expected network resources are available.

• No issues found!
```

</details>