[Pre6] `NotFound` works with streaming that has started

[ilonatommy]

PR: dotnet/aspnetcore#62178.

There is a change to how streaming scenarios where response started handle the rendering.

If enhanced navigation is not blocked, it is used to render NotFound contnet, otherwise they keep the old behavior of redirecting to NotFound content. NotFound content is defined as NotFoundPage if passed to Router, otherwise as re-execution page when the re-execution middleware is set. Streaming that has started does not support rendering NotFound fragment and it is recommended not to use it in net 10.