CmdPal: Fix filter separators (two in one) (#41834)

## Summary of the Pull Request

Drop-in fix for two issues with search filter separators:
- Separator visual template is not applied when AOT compilation is
enabled.
- Separator template uses a different brush then other separators.

Pictures? Pictures!

<img width="935" height="1178" alt="image"
src="https://github.com/user-attachments/assets/d4fcb5a8-1610-4972-adc3-9f301cb2ed50"
/>


## PR Checklist

- [x] Closes: #41441 
- [ ] **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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## 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

src/modules/cmdpal/Microsoft.CmdPal.UI/Controls/FiltersDropDown.xaml

@@ -59,7 +59,7 @@
                 <Rectangle
                     Height="1"
                     Margin="-16,-12,-12,-12"
-                    Fill="{ThemeResource MenuFlyoutSeparatorThemeBrush}" />
+                    Fill="{ThemeResource MenuFlyoutSeparatorBackground}" />
             </DataTemplate>
 
         </ResourceDictionary>