ShadeEffect crashes with unhandled error with a ThemeResource

### Describe the bug

Assigning a ThemeResource in XAML for the Color property of ShadeEffect throws the following exception:

Exception thrown: 'System.Runtime.InteropServices.COMException' in System.Private.CoreLib.dll
WinRT information: Failed to assign to property 'CommunityToolkit.WinUI.UI.Media.ShadeEffect.Color'. [Line: 316 Position: 48]
An exception of type 'System.Runtime.InteropServices.COMException' occurred in System.Private.CoreLib.dll but was not handled in user code

Using a StaticResource works great.

The following XAML code will reproduce the issue:

```

```

### Regression

_No response_

### Reproducible in sample app?

- [X] This bug can be reproduced in the sample app.

### Steps to reproduce

```text
1. Create a new WinUI application
2. Replace the XAML for MainWindow with the one in the description above
3. Start the application
```

### Expected behavior

Expected theme resource to "just work" the same as a static resource.

### Screenshots

_No response_

### 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)
- [X] 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)
- [X] Windows 10, version 2004 (Build 19041)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

2022

### Visual Studio Build Number

_No response_

### Device form factor

Desktop

### Nuget packages

CommunityToolkit.WinUI.UI.Media 7.1.2

### Additional context

_No response_

### Help us help you

No.

Start by running the sample app and reproducing the failure from MainWindow's XAML, using the ThemeResource assignment to ShadeEffect.Color. Compare it with the working StaticResource cases and inspect the media effect entry points involved. Done means the ThemeResource example starts without an unhandled COMException and the existing cases still work.

Scritto dal modello di indicizzazione a partire dal testo della issue.