Enumerating theme resources breaks theming

### Describe the bug

When application's theme dictionaries are enumerated, dynamic theme changes stop working.

### Why is this important?

Enumerating should not have side-effects.

### Steps to reproduce the bug

1. Blank WinUI app
2. XAML:

```

Change theme
Enumerate

```

3. Code-behind

```csharp
private void ChangeTheme()
{
ParentGrid.RequestedTheme = ElementTheme.Dark;
}

private void EnumerateRDs()
{
foreach (var kvp in Application.Current.Resources.MergedDictionaries)
{
foreach (var th in kvp.ThemeDictionaries)
{
if (th.Value is ResourceDictionary r)
{
foreach (var key in r)
{
}
}
}
}
}
```

4. Have OS in light theme, run app and click Change theme button - notice the theme changes successfully and background changes to dark
5. Stop debugging
6. Start app again and click Enumerate button
7. **Click Change theme button and notice theme no longer changes and background stays light.**

### Actual behavior

`RequestedTheme` has no effect

### Expected behavior

Should work

### Screenshots

_No response_

### NuGet package version

1.8.260317003

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

Reproduce the issue in a blank WinUI app using the provided XAML and the ChangeTheme and EnumerateRDs handlers. Start by examining Application.Current.Resources.MergedDictionaries and its ThemeDictionaries during enumeration. Done means enumerating the resource dictionaries does not prevent a later RequestedTheme change from updating the background.

Written by the indexing model from the issue text.