fix(model_switch): register custom: slug in seen_slugs for Section 3 providers

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>

hermes_cli/model_switch.py

@@ -1095,6 +1095,7 @@ def list_authenticated_providers(
                 "api_url": api_url,
             })
             seen_slugs.add(ep_name.lower())
+            seen_slugs.add(custom_provider_slug(display_name).lower())
             _pair = (
                 str(display_name).strip().lower(),
                 str(api_url).strip().rstrip("/").lower(),