Issue #3605518: Keep menu links to entities on another site visible

Menu links pointing to a site-scoped entity on a different site were removed by the default menu tree access check, which runs entity access that the sites module forbids for cross-site entity view. Decorate menu.default_tree_manipulators and pre-grant access to links carrying a site_id/site option for a different, existing site, before the default check wraps them in an InaccessibleMenuLink. Such links already render as absolute URLs to the target site via SitesUrlGenerator, so no entity access is bypassed. Link fields need no change (core LinkFormatter does not access-check the URL). Add unit coverage for the manipulator.

Closes #3605518