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.

まず、page.xaml.cs、MyViewModel.cs、MyObject.cs にあるネストされた ViewModel.Object.Tags の例を使って、page.xaml の binding を再現します。TokenizingTextBox の ItemSource binding を追跡し、ネストされた ObservableCollection が InterspersedObservableCollection にキャストされる理由を特定します。同じ binding で InvalidCastException がスローされなくなれば完了です。

索引モデルが issue の本文から書いたものです。