Fix HostBuilderHandlerTests for net11.0 IMauiContext parameter requirement

[Copilot]

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Description of Change

The IMauiHandlersFactory.GetHandler API now requires an IMauiContext parameter for handler resolution. Updated all test methods in HostBuilderHandlerTests.cs to create and pass the required context:

var mauiHandlersFactory = mauiApp.Services.GetRequiredService<IMauiHandlersFactory>();
var context = new HandlersContextStub(mauiApp.Services);

// Now requires context parameter
var handler = mauiHandlersFactory.GetHandler<IViewStub>(context);
var handler = mauiHandlersFactory.GetHandler(typeof(IViewStub), context);

Changes:

• Added IMauiContext instantiation via HandlersContextStub in all affected test methods
• Updated 11 test methods to pass context to GetHandler calls

All 23 tests in HostBuilderHandlerTests now pass with the updated handler resolution API.

Issues Fixed

Part of #29952 - net11.0 branch build failures after ElementHandlerAttribute changes.

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