Community verification of #56688 (zmack12344321) found two follow-up gaps
that kept Vertex invisible in the /model menu even after registry
registration:
1. hermes_cli/model_switch.py: list_authenticated_providers() had a
credential gate hard-coded to API keys (with an aws_sdk special case
only) — add a vertex branch using has_vertex_credentials(), mirroring
the aws_sdk shape.
2. hermes_cli/models.py: Vertex's OpenAI-compatible endpoint has no
/models listing route, so without a curated _PROVIDER_MODELS entry the
picker only ever showed the current model — add a Gemini curated list.
Follow-up to #56688.
