UnhandledException when using ResourceDictionary with CustomResource and WindowsAppSDK 1.7.25

### Describe the bug

Custom class defined in xaml constructs new CustomXamlResourceLoader in its static constructor. In xaml the setup is the following:

```xaml

```

In the constructor of `UserThemeResources` we set:

```cs
CustomXamlResourceLoader.Current = new UserThemeResourceLoader();
```

_This builds and runs fine with WindowsAppSDK **1.6.25**._

In 1.7.256 version of SDK we get:

```
{Microsoft.UI.Xaml.UnhandledExceptionEventArgs}
Exception: {"The text associated with this error code could not be found.\r\n\r\nNo custom resource loader set. [Line: 17 Position: 27]"}
```

and also the static constructor of UserThemeResources is not fired at all.

### Steps to reproduce the bug

[UserThemeResources_Test.zip](https://github.com/user-attachments/files/19651603/UserThemeResources_Test.zip)

Run the application with WindowsAppsSDK 1.7.25 restored. Notice the exception

### Expected behavior

No exception and the constructor to fire.

### Screenshots


### NuGet package version

None

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

Start with the attached UserThemeResources_Test.zip and reproduce the failure using WindowsAppSDK 1.7.25, then compare the same resource-loading setup with 1.6.25. Inspect the UserThemeResources initialization and CustomXamlResourceLoader.Current entry points shown in the report; done means the resource dictionary loads without an unhandled exception and the loader constructor is reached.

Written by the indexing model from the issue text.