CmdPal: Hide separator between search bar and context grids when collapsed (#49313)

## Summary of the Pull Request

This PR updates a conditional visibility of a border/separator line
between search bar and page content to hide it when the ShellPage is in
collapsed mode.

## Pictures? Pictures!

<img width="472" height="108" alt="image"
src="https://github.com/user-attachments/assets/eebc6be2-7dbf-4861-812e-d06ab1d42257"
/>

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## PR Checklist

- [x] Closes: #49312
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issues) -->
- [ ] **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

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## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/BindTransformers.cs

@@ -27,4 +27,7 @@ internal static class BindTransformers
 
     public static Visibility VisibleWhenAny(bool value1, bool value2)
         => (value1 || value2) ? Visibility.Visible : Visibility.Collapsed;
+
+    public static Thickness BottomBorderThicknessWhenAll(bool value1, bool value2)
+        => value1 && value2 ? new Thickness(0, 0, 0, 1) : new Thickness(0);
 }

src/modules/cmdpal/Microsoft.CmdPal.UI/Pages/ShellPage.xaml

@@ -206,7 +206,7 @@
                 HorizontalAlignment="Stretch"
                 VerticalAlignment="Stretch"
                 BorderBrush="{ThemeResource CmdPal.TopBarBorderBrush}"
-                BorderThickness="0,0,0,1">
+                BorderThickness="{x:Bind h:BindTransformers.BottomBorderThicknessWhenAll(ViewModel.IsSearchBoxVisible, ExpandedMode), Mode=OneWay}">
                 <Grid.ColumnDefinitions>
                     <ColumnDefinition Width="Auto" />
                     <ColumnDefinition Width="*" />
@@ -596,7 +596,6 @@
                     <VisualState.Setters>
                         <Setter Target="SearchBox.IsEnabled" Value="False" />
                         <Setter Target="SearchBox.Opacity" Value="0" />
-                        <Setter Target="TopBarGrid.BorderBrush" Value="Transparent" />
                     </VisualState.Setters>
                 </VisualState>
             </VisualStateGroup>