Add validation to Router component to prevent both NotFound and NotFoundPage being set

[Copilot]

In this PR:

• Add nullability operator in type for Router.NotFoundPage
public Type NotFoundPage { get; set; } -> public Type? NotFoundPage { get; set; }
• Mark NotFound render fragment in Router as deprecated.
• When both: NotFound and NotFoundPage are defined, log a warning. Do it in Router.SetParametersAsync in if (NotFoundPage != null) condition. Use the logger that is available there, add a new method to private static partial class Log with warning level. In the warning message, underline that we prefer NotFoundPage to be used and that NotFound will be deprecated. We are disabling treating warnings as errors, so they have to be disabled in some places to fix the build.

Fixes: #62409

[ilonatommy]

@copilot, add one unit test to RouterTest.cs to check if the warning is produced correctly when we have both: NotFound fragment and NotFoundPage.

[Copilot]

@copilot, add one unit test to RouterTest.cs to check if the warning is produced correctly when we have both: NotFound fragment and NotFoundPage.

Added unit test LogsWarningWhenBothNotFoundAndNotFoundPageAreSet to RouterTest.cs that verifies the warning is logged correctly when both NotFound fragment and NotFoundPage are set. The test uses a custom TestLogger to capture the warning message and validates both the warning level and message content. Commit: aa54562