ResourceLoader.GetString throws COMException (NamedResource Not Found) for keys containing '.' in WinUI 3 desktop app

### Describe the bug

When calling:
```cs
var loader = new ResourceLoader();
var value = loader.GetString("XXXXX.Common.Yes");
```
a COMException is thrown:
```
NamedResource Not Found.
```
The resource key does exist in Resources.resw.

If the key is changed from:
```
XXXXX.Common.Yes
```
to:
```
XXXXX_Common_Yes
```
the lookup succeeds immediately.

This indicates that resource keys containing . may not be resolved correctly by ResourceLoader.GetString.

The exception originates from WinRT resource lookup and is reported by Visual Studio at throw time (even if later caught).

API reference:
https://learn.microsoft.com/windows/windows-app-sdk/api/winrt/microsoft.windows.applicationmodel.resources.resourceloader.getstring
https://github.com/microsoft/WindowsAppSDK/blob/main/dev/MRTCore/mrt/Microsoft.Windows.ApplicationModel.Resources/src/ResourceLoader.cpp

### Steps to reproduce the bug

Create a new WinUI 3 Desktop project (C#/C++).

Add a string resource in Resources.resw:
```
Name: XXXXX.Common.Yes
Value: Yes
```
Call:
```cs
var loader = new ResourceLoader();
var value = loader.GetString("XXXXX.Common.Yes");
```
Observe that COMException (NamedResource Not Found) is thrown.

Change the key to:
```
XXXXX_Common_Yes
```
Update the code accordingly and rerun.

Observe that the lookup succeeds.

### Expected behavior

WinUI 3 (via Windows App SDK)

Desktop application

Note: This behavior was observed in a desktop project. Packaged vs. unpackaged scenarios have not been independently verified, so no conclusion is made regarding packaging impact.

### Screenshots

### NuGet package version

Windows App SDK 1.8.5: 1.8.260209005

### Packaging type

Unpackaged

### Windows version

Insider Build (xxxxx)

### IDE

_No response_

### Additional context

Replacing . with _ consistently resolves the issue.

This may relate to URI fragment semantics or MRT/PRI key normalization.

Clarification would be appreciated on whether . is officially supported in resource keys for ResourceLoader.GetString.

Start by reproducing the lookup with a dotted and underscored key in an unpackaged WinUI 3 desktop project using Windows App SDK 1.8.5. Read the ResourceLoader.GetString API reference and dev/mrt/mrt/Microsoft.Windows.ApplicationModel.Resources/src/ResourceLoader.cpp; done means establishing whether dotted keys are supported and documenting or correcting the observed behavior.

Written by the indexing model from the issue text.