[6.x] Fix site URLs configured with Antlers being treated as external

[duncanmcclean]

This pull request fixes an issue where URLs on a site's own host were left untouched by URL::tidy() when the site's URL is configured with Antlers, keeping trailing slashes on permalinks, canonical URLs, sitemap entries, etc. This mostly affects headless setups, where the site URLs (eg. {{ config:app:frontend_url }}/fr) point at a different host than APP_URL.

This was happening because URL::ensureSiteCaches() built its list of known site hosts from rawConfig()['url'] — the unresolved config value. An Antlers value like {{ config:app:frontend_url }}/fr isn't an absolute URL, so it was filtered out of the list and the site's host was never recognised as belonging to the application. Single-host setups were unaffected because the host also matches config('app.url'), which is checked separately.

This PR fixes it by building the site caches from $site->url() — the resolved config value the Site object has already computed by that point, so there's no extra parsing involved.

Fixes #15232

[jasonvarga]

The usage of $site->rawConfig()['url'] looked intentional to me. But I can't find a reason for why, and AI tried to test for recursion or performance issues and it found nothing either.