Unify `NotFound` behavior across render modes

[ilonatommy]

Problems that we would like to solve:

• re-execution takes precedence over NotFound rendering,
• url of page is changed when we're streaming, having the blazor's router,
• not enough tests for each case, especially custom router tests.

This table presents current implementation for each combination of settings (Description) and actions to fix them (ToDo):

Application Interactivity	Content render mode	Default router	Description	ToDo
Global	SSR non-streaming	With	Renderer is setting the status code to 404 and stopping the rendering. Router tries to render 404 fragment but the component invoker decides to finish rendering early and not to flush the response because it expects re-execution. The only way to show 404 page is to re-execute, otherwise we would see a default browser’s not found page:	We should flush the response during rendering in component invoker regardless of returned code and re-execution middleware settings.
Per-component	SSR non-streaming	With	Same situation as global interactivity.	Row 1 solves it.
Global	SSR streaming	With	The status code is already set; we cannot change it. Instead, we inject redirect command into JS with constant “not-found” route. We stop the renderer.	We should not try to change the code nor JS-redirect. We should enable streaming in Router and allow it render the rest of the page.
Per-component	SSR streaming	With	Same situation as global interactivity.	Row 3 solves it.
Global	Interactive	With	Same situation as 1st and 2nd scenario.	Row 1 solves it.
Per-component	Interactive	With	Impossible combination. We cannot render interactive page, not having interactivity registered for the application.	-
Any	SSR non-streaming	Without	The only way to render 404 page is to re-execute.	No action needed.
Any	SSR streaming	Without	Not tested.	We should advice users with custom routers to enable streaming on them. In case they don't want to do it, we should add instructions in endpoint renderer to the response on how to redirect to "not found" page and read them in JS-side of streaming renderer. JS should do it only if Router did not flag NotFound as handled, e.g. in the event arguments.