External subscribers can set NotFoundEventArgs.Path

Connected with risks listed in #62412. In this PR, the listed scenarios are treated as supported, excluding case "we have no router and the request has not started" where we are not able to render the path set by external subscriber and only re-execution could render the not found contents.

This PR focuses on fixing scenario "we have blazor's router and the request has not started, we allow the Router to render only if it has NotFoundPage parameter passed". It also adds tests for all the scenarios mentioned in the linked issue.

Description

• Previously Router ignored all event calls if NotFoundPage was not populated. That included a case of external subscriber that set NotFoundEventArgs.Path.
• We're changing the condition to treat populated args.Path as a request to render the contents in this path. In that case, Router is able to find a component type by route and render it.
• [Docs] The update adds flexibility: application can either set Router.NotFoundPage or NotFoundEventArgs.Path. If both are defined, we should respect NotFoundEventArgs.Path because it's easier to overwrite it. It can be used, e.g. like this:

NavigationManager.OnNotFound += (sender, args) =>
{
    // Show different not-found pages based on the attempted URL
    if (NavigationManager.BaseUri.StartsWith("/admin/"))
        args.Path = "/admin-not-found";
    else if (NavigationManager.BaseUri.StartsWith("/api/"))
        args.Path = "/api-not-found";
    else if (NavigationManager.BaseUri.StartsWith("/user/"))
        args.Path = "/user-not-found";
    // Otherwise, let the default `NotFoundPage` handle it
};

• [Docs] For the scenario to work, external subscriber has to subscribe to NavigationManager.OnNotFound before Router does it. In case of custom router, it's enough to do it in its OnParametersSet/Async, in case of default Router, it's best to do it in App.razor.