CmdPal: Add hidden window as owner for tool windows (#42902)

## Summary of the Pull Request

This PR changes the method used to hide tool windows from the taskbar
and Alt+Tab to a more reliable approach.
Previously, this was achieved by adding `WS_EX_TOOLWINDOW` to an unowned
top-level window, which proved unreliable in several scenarios.

The new implementation assigns a hidden window as the owner of each tool
window.
This ensures that the window does not appear on the taskbar even when
the Windows setting
**Settings → System → Multitasking → On the taskbar, show all opened
windows** is set to **On all desktops**.

## Change log one-liner

Fixes Command Palette windows occasionally appearing on the taskbar
under certain system settings.

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] Closes: #42395
- [x] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [x] **Tests:** Added/updated and all pass
- [x] **Localization:** All end-user-facing strings can be localized
- [x] **Dev docs:** Added/updated
- [x] **New binaries:** none
- [x] **Documentation updated:** no need

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

Tested alongside the stable CmdPal on a system with

src/modules/cmdpal/Microsoft.CmdPal.UI/ToastWindow.xaml.cs

@@ -25,11 +25,11 @@ public sealed partial class ToastWindow : WindowEx,
     IRecipient<QuitMessage>
 {
     private readonly HWND _hwnd;
+    private readonly DispatcherQueueTimer _debounceTimer = DispatcherQueue.GetForCurrentThread().CreateTimer();
+    private readonly HiddenOwnerWindowBehavior _hiddenOwnerWindowBehavior = new();
 
     public ToastViewModel ViewModel { get; } = new();
 
-    private readonly DispatcherQueueTimer _debounceTimer = DispatcherQueue.GetForCurrentThread().CreateTimer();
-
     public ToastWindow()
     {
         this.InitializeComponent();
@@ -39,12 +39,7 @@ public sealed partial class ToastWindow : WindowEx,
         this.SetIcon();
         AppWindow.Title = RS_.GetString("ToastWindowTitle");
         AppWindow.TitleBar.PreferredHeightOption = TitleBarHeightOption.Collapsed;
-
-        // Tool windows don't show up in ALT+TAB, and don't show up in the taskbar
-        // Since tool windows have smaller corner radii, we need to force the normal ones
-        // to visually match system toasts.
-        this.ToggleExtendedWindowStyle(WINDOW_EX_STYLE.WS_EX_TOOLWINDOW, true);
-        this.SetCornerPreference(DWM_WINDOW_CORNER_PREFERENCE.DWMWCP_ROUND);
+        _hiddenOwnerWindowBehavior.ShowInTaskbar(this, false);
 
         _hwnd = new HWND(WinRT.Interop.WindowNative.GetWindowHandle(this).ToInt32());
         PInvoke.EnableWindow(_hwnd, false);