Two fixes for user-defined providers in config.yaml:
1. list_authenticated_providers() - now includes full models list from
providers.*.models array, not just default_model. This fixes /model
showing only one model when multiple are configured.
2. _get_named_custom_provider() - now checks providers: dict (new-style)
in addition to custom_providers: list (legacy). This fixes credential
resolution errors when switching models via /model command.
Both changes are backwards compatible with existing custom_providers list format.
Fixes: Only one model appears for custom providers in /model selection
