Unexpected "InputSiteWindowClass" window appears in WinUI 3 app

### Describe the bug

I am experiencing an issue in my WinUI 3 application where an unexpected window with the class name InputSiteWindowClass appears, even though I am only initializing the main Window. This window is not explicitly created or referenced in my code, and it seems to be related to internal framework behavior, possibly related to input handling or focus management.

This issue doesn't occur in a similar app built using the same version of WinUI 3, which makes me believe it could be a bug related to how the framework handles certain window classes.

### Steps to reproduce the bug

Create a basic WinUI 3 app (or use the sample app from GitHub).

Launch the app and inspect the UI hierarchy using Inspect.exe or Spy++.
Observe that an unexpected window with the class name InputSiteWindowClass appears, which is not explicitly instantiated in the code.

### Expected behavior

Only the main Window should appear with the specified content. The InputSiteWindowClass should not be instantiated unless it is a part of the framework’s internal handling of input, which should be transparent to the user.

### Screenshots


### NuGet package version

WinUI 3 - Windows App SDK 1.6.3: 1.6.241114003

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

The unexpected window is detected by Testing Tool and appears as a greyed-out object.
Tried debugging the code and checking for window creation in OnLaunched, but no explicit reference to InputSiteWindowClass is found.
Similar behavior is observed in a fresh WinUI 3 app that follows the same structure, but not in another app built with WinUI 3.
Other tools like Inspect.exe and Spy++ also show this window in the UI tree.

Start by reproducing the issue in a basic WinUI 3 app or the referenced sample using Windows App SDK 1.6.3 on Windows 11 24H2, then inspect the UI hierarchy with Inspect.exe or Spy++. Compare the observed InputSiteWindowClass behavior with the similar WinUI 3 app; done means establishing whether this framework window is expected internal behavior or an actionable bug.

Written by the indexing model from the issue text.