[6.x] Fix duplicate slugs allowed with depth-conditional routes

[duncanmcclean]

This pull request fixes an issue where duplicate slugs were allowed for child entries in structured collections when using a route configuration that references depth (e.g. {{ if depth > 1 }}{{ parent_uri }}/{{ slug }}{{ else }}base/{{ slug }}{{ /if }}).

This was happening because the depth variable was not being passed to the URL builder during unique URI validation in EntriesController::entryUri(). There was a // 'depth' => '', // todo placeholder where it should have been calculated. Without depth, the Antlers conditional in the route couldn't evaluate correctly, so the generated URI during validation didn't match the actual URI, and duplicates weren't detected.

This PR fixes it by calculating the depth from the parent page's depth and passing it to the URL builder.

Fixes #14482