Add NotFoundPage to Router

[ilonatommy]

Description

• Adds NotFoundPage parameter that can be used to customize the view of page rendered on NotFoundEvent.
• Fixes the configuration without global router: then no subscription to _notFound was done, so we use redirection to a hardcoded not-found page.
• Fixes the SSR streaming configuration. In streaming scenario we typically want to set 404 after the response already started (e.g. after request to a db). Throwing an exception in this case does not look good. Instead, we ignore the 404 there and redirect to NotFound page.
• If SSR streaming did not start or is not enabled, we can reexecute the request and set the result url to a chosen path, using app.UseStatusCodePagesWithReExecute("/reexecution-path", createScopeForErrors: true);

Fixes #58815

[javiercn]

Looks great!

I think we will also want to update the project templates to use this feature.

In addition to it, I think there's another scenario we want to enable here which is to make sure that it works with UseStatusCodeWithReexecute

• UseStatusCodeWithReexecute allows the app to render a page after a 404 has been set somewhere. We want that to also be possible with a Component page

I did something similar for handling errors a couple releases back. This is the PR with the change for reference #50550

• I suspect we will need to do something similar here.

/cc: @danroth27

[ilonatommy]

This is an improvement in testing templates and a bit that I was missing to debug the results of *Locally scripts. It could be a separate PR if really needed.

[javiercn]

Didn't realize this, but since NotFoundEventArgs doesn't have any arguments, we can reuse the same instance privately (like Array.Empty()`) and save allocations

[javiercn]

Typo: requiresReexecution

Suggested change

	bool requresReexecution = originalStatusCode != context.Response.StatusCode && hasStatusCodePage;
	if (requresReexecution)
	{
	// If the response is a 404, we don't want to write any content.
	// This is because the 404 status code is used by the routing middleware
	// to indicate that no endpoint was found for the request.
	await bufferWriter.FlushAsync();
	return;
	}
	bool requiresReexecution = originalStatusCode != context.Response.StatusCode && hasStatusCodePage;
	if (requiresReexecution)
	{
	// If the response is a 404, we don't want to write any content.
	// This is because the 404 status code is used by the routing middleware
	// to indicate that no endpoint was found for the request.
	await bufferWriter.FlushAsync();
	return;
	}

[javiercn]

You could combine these two conditions into a variable at the beginning and reuse that through the remainder of the method, it seems that they are used in at least a couple of places

[javiercn]

Will this cause the NotFound page to be rendered during streaming rendering?

[javiercn]

I don't think we want to include the line breaks in the log. If you are looking to make this readable, you can break it using + instead.