ToolkitSampleMultipleChoiceOption broken

### Describe the bug

The ``ToolkitSampleMultiChoiceOption`` is unable to parse data.

The error

The sample choices

The sample XAML

### Steps to reproduce

```text
Use https://github.com/Avid29/Labs-Windows/tree/marquee
```

### Expected behavior

The strings should be parsed for XAML binding

### Screenshots

_No response_

### Code Platform

- [ ] UWP
- [ ] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

_No response_

### Visual Studio Build Number

_No response_

### Device form factor

_No response_

### Additional context

This is breaking my PR for CommunityToolkit/Labs-Windows#275

### Help us help you

No, I'm unable to contribute a solution.

Beginne damit, den in den Reproduktionsschritten referenzierten Labs-Windows-marquee-Branch zu prüfen und die Implementierung von ToolkitSampleMultiChoiceOption sowie deren Beispiel-XAML zu finden. Reproduziere den Parsing-Fehler und überprüfe anschließend, dass die Beispielauswahlzeichenfolgen für die XAML-Bindung korrekt geparst werden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.