Views display plugins with contextual filters produce invalid paths/URLs

When a Views display uses a contextual filter that uses a slug in the path (e.g. /node/%node/stuff), the results from getPath() and getUrl() are garbage and lead to issues when used. There was already acknowledgement of this shortcoming in one form in #2842557: Search API sorts doesnt work with contextual filters, but it only addressed the implementation of isRenderedInCurrentRequest().

Original issue (QueryString processor causes 404 for Views page displays that have upconverted contextual filter)

This one caused my head to hurt running it down. :)

Say you have a facet whose source is a Views page display. The path for this display includes a contextual filter AND the slug makes use of automatic upconversion, e.g. /user/%user/stuff.

The QueryString processor tries to build URLs by creating a request from the facet source's path.

    if ($facet->getFacetSource()->getPath()) {
      $request = Request::create($facet->getFacetSource()->getPath());
    }
    ...
    $requestUrl = Url::createFromRequest($request);

When creating a URL from the request, this blows up because the route enhancer that converts the parameter to the full entity barfs (%user is not a valid user ID). Since it can't load the entity, the ParamConverterManager throws an exception that in turn causes a 404 to be issued.