Binding TokenizingTextBox ItemSource to a nested model throws InvalidCastException

### Describe the bug

Binding the TokenizingTextBox ItemSource to an ObservableCollection inside a nested object eg. {x:Bind ViewModel.MyObject.Tags, Mode=TwoWay} results in InvalidCastException: Unable to cast object of type 'CommunityToolKit.WinUi.Controls.InterspersedObservableCollection' to type 'System.Collections.ObjectModel.ObservalbeCollection[System.String]'

### Steps to reproduce

```text
page.xaml
...

...

page.xaml.cs
...
public MyViewModel ViewModel {get;}
...

MyViewModel.cs
...
public MyObject Object {get; set;} = new();
...

MyObject.cs
...
public ObservableCollection Tags {get; set;} = new();
...
```

### Expected behavior

No exception should be thrown

### Screenshots

_No response_

### Code Platform

- [ ] UWP
- [x] 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 10 21H2 (Build 19044)
- [ ] Windows 10 22H2 (Build 19045)
- [x] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [x] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Windows 10, version 2104 (Build 20348)
- [ ] Windows 11, version 22H2 (Build 22000)
- [x] Other (specify)

### Other SDK version

22621

### Visual Studio Version

_No response_

### Visual Studio Build Number

_No response_

### Device form factor

_No response_

### Additional context

_No response_

### Help us help you

No, I'm unable to contribute a solution.

Start by reproducing the binding in page.xaml using the nested ViewModel.Object.Tags example from page.xaml.cs, MyViewModel.cs, and MyObject.cs. Trace the TokenizingTextBox ItemSource binding to identify why the nested ObservableCollection is cast to InterspersedObservableCollection. Done means the same binding no longer throws InvalidCastException.

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