[ci-scan-net11] Windows TabbedPage device tests fail to compile because AssertEventually is missing

## Summary
The Windows device-test build fails before tests run because `TabbedPageTests.Windows.cs` references an unavailable `AssertEventually` helper.

## Build Information
- **Pipeline**: maui-pr
- **Build**: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1567261
- **Build ID**: 1567261
- **Branch**: net11.0
- **First seen**: 2026-08-25
- **Occurrences**: 1 in last 10 builds

## Affected Legs
- Windows 10.0.19041.0 device tests
- Windows 10.0.20348.0 device tests

## Error Message
```
2026-08-25T17:44:48.3704035Z D:\a\_work\1\s\src\Controls\tests\DeviceTests\Elements\TabbedPage\TabbedPageTests.Windows.cs(141,11): error CS0103: The name 'AssertEventually' does not exist in the current context [D:\a\_work\1\s\src\Controls\tests\DeviceTests\Controls.DeviceTests.csproj::TargetFramework=net11.0-windows10.0.19041.0]
2026-08-25T18:15:39.9263031Z D:\a\_work\1\s\src\Controls\tests\DeviceTests\Elements\TabbedPage\TabbedPageTests.Windows.cs(141,11): error CS0103: The name 'AssertEventually' does not exist in the current context [D:\a\_work\1\s\src\Controls\tests\DeviceTests\Controls.DeviceTests.csproj::TargetFramework=net11.0-windows10.0.19041.0]
```

## Recommended Action
Update `TabbedPageTests.Windows.cs` to use an available eventual-assert helper or add the required helper import/implementation, then compile both Windows target frameworks.

Start in src/Controls/tests/DeviceTests/Elements/TabbedPage/TabbedPageTests.Windows.cs at the AssertEventually reference around line 141, then inspect the available helpers and imports used by the device tests. Update the reference or provide the required helper, and verify that Controls.DeviceTests.csproj compiles for both Windows target frameworks shown in the failure.

Written by the indexing model from the issue text.