custom_providers entries (section 4 of list_authenticated_providers) only
read the static models: dict from config.yaml, ignoring the live /v1/models
endpoint. This means gateways like Bifrost that expose hundreds of models
only show the handful explicitly listed in config.
Add live discovery via fetch_api_models() for custom_providers entries
that have api_key + base_url, matching the existing behavior for user
providers: entries (section 3). When the endpoint is reachable and
returns models, the live list replaces the static subset.
Fixes: /model picker showing only 9 models from a Bifrost gateway that
actually exposes 581.
abe5a3c937