A providers: entry with only a default_model/model (no explicit models:
list) is un-narrowed — the singular field is just the active selection.
Section 3 derived has_explicit_models from the merged models list, so
the lone default_model entry counted as an explicit catalog and
suppressed the /v1/models probe for no-key endpoints, leaving a
one-line /model picker menu for local llama.cpp/Ollama/vLLM servers.
Track explicit models: declarations separately at group-build time
(mirrors section 4's declaration-tracking from #40542 / PR #61928) and
gate the probe on that instead.
Salvaged from PR #68984 by @vigilancetech-com (the probe_custom_providers
gate removal in that PR is not taken — the GUI no-probe gate is
intentional).
