[net11.0] Fix Windows build: add missing 'using static AssertHelpers' to TabbedPageTests.Windows.cs (#37818)

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## Description

PR #37755 added Windows TabbedPage badge tests that call
`AssertEventually`, but `TabbedPageTests.Windows.cs` was missing the
static import for `Microsoft.Maui.DeviceTests.AssertHelpers`. This
causes CS0103 in both net11.0 Windows TFMs and blocks the Debug and
Release Windows builds for every PR targeting `net11.0`.

This adds the same `using static
Microsoft.Maui.DeviceTests.AssertHelpers;` directive already present in
every sibling `TabbedPageTests` partial. No test logic or runtime
behavior changes.

## Verification

Verified by code inspection that the sibling partials use the same
static import and that `AssertHelpers.AssertEventually(Func<bool>, ...)`
exists. Windows compile verification is deferred to CI because the local
host is macOS and cannot build the `net11.0-windows` TFMs.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: mattleibow

src/Controls/tests/DeviceTests/Elements/TabbedPage/TabbedPageTests.Windows.cs

@@ -13,6 +13,7 @@
 using Microsoft.Maui.Hosting;
 using Microsoft.Maui.Platform;
 using Xunit;
+using static Microsoft.Maui.DeviceTests.AssertHelpers;
 using WContentPresenter = Microsoft.UI.Xaml.Controls.ContentPresenter;
 using WFrame = Microsoft.UI.Xaml.Controls.Frame;
 using WFrameworkElement = Microsoft.UI.Xaml.FrameworkElement;