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.

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## 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

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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/NativeMethods.txt

@@ -58,4 +58,9 @@ GetModuleHandle
 
 GetWindowLong
 SetWindowLong
-WINDOW_EX_STYLE
+WINDOW_EX_STYLE
+CreateWindowEx
+WNDCLASSEXW
+RegisterClassEx
+GetStockObject
+GetModuleHandle