Section 3 (user-defined endpoints) added the plain ep_name to seen_slugs
but not the custom:-prefixed slug. Section 4 generates custom:<name> via
custom_provider_slug() and checks seen_slugs — since the prefixed slug
was missing, the same provider appeared twice in /model.
Register custom_provider_slug(display_name).lower() in seen_slugs after
Section 3 emits a provider, so Section 4's dedup correctly suppresses
the duplicate.
Closes#12293.
Co-authored-by: bennytimz <bennytimz@users.noreply.github.com>
c1977146ce