Return stub IAlertManagerSubscription for standard AlertManager

[drasticactions]

.NET 10 MAUI introduced a new internal implementation of IAlertManagerSubscription. This includes a standard implementation with a NotImplementedException for CreateSubscription.

maui/src/Controls/src/Core/Platform/AlertManager/AlertManager.Standard.cs

Lines 8 to 11 in 3680b04

	private partial IAlertManagerSubscription CreateSubscription(IMauiContext mauiContext)
	{
	throw new NotImplementedException();
	}

This creates a problem if you are using .NET implementations of the controls from outside the standard .NET MAUI UI TFMs (Such as test cases for controls outside of the MAUI repo). Window calls on an internal AlertManager implementation,

maui/src/Controls/src/Core/Window/Window.cs

Lines 696 to 700 in 3680b04

	void OnPageHandlerChanged(object? sender, EventArgs e)
	{
	ModalNavigationManager.PageAttachedHandler();
	AlertManager.Subscribe();
	}

This calls on that standard AlertManager, which then fails with a not implemented exception. in .NET 9 and previous, this would no-op.

Returning the stub AlertManagerHelper fixes this and lets it continue. For 99.999% of users (scientifically speaking), they shouldn't notice a difference in behavior.

[dotnet-policy-service]

Hey there @@drasticactions! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Copilot]

Pull Request Overview

This PR fixes a regression in .NET 10 MAUI where the standard AlertManager implementation throws NotImplementedException when CreateSubscription is called. This breaks test scenarios where MAUI controls are used outside the standard MAUI UI TFMs without dependency injection services configured.

Key changes:

• Returns a no-op AlertRequestHelper stub instead of throwing NotImplementedException in the standard AlertManager
• Aligns with .NET 9 behavior where this scenario would silently no-op
• Matches the pattern of platform-specific implementations (Android, iOS, Windows, Tizen) which all return concrete instances