CmdPal: Make Command Palette's main and toast windows tool windows (#41835)

## Summary of the Pull Request
Replaces the `SetVisibilityInSwitchers` method with `WS_EX_TOOLWINDOW`
style, removing reliance on the fragile `IsShownInSwitchers` property.
Explicitly overrides window corner preference so tool windows keep the
same rounded corners as regular app windows (tool windows otherwise
default to a smaller corner radius, which would change the look).

Benefits:
- Avoids use of the unreliable `IsShownInSwitchers` property, which has
multiple known failure cases.
- Allows window managers to correctly treat Command Palette as a popup
window rather than a normal app window, preventing unwanted tiling, grid
placement, or similar behaviors. This better matches the intended design
of CmdPal as an overlay UI.


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## PR Checklist

- [x] Closes: #41772
- [x] Closes: #41593
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/NativeMethods.txt

@@ -44,6 +44,7 @@ MessageBox
 DwmGetWindowAttribute
 DwmSetWindowAttribute
 DWM_CLOAKED_APP
+DWM_WINDOW_CORNER_PREFERENCE
 
 CoWaitForMultipleObjects
 INFINITE
@@ -53,4 +54,8 @@ GetCurrentThreadId
 SetWindowsHookEx
 UnhookWindowsHookEx
 CallNextHookEx
-GetModuleHandle
+GetModuleHandle
+
+GetWindowLong
+SetWindowLong
+WINDOW_EX_STYLE