When an anonymous custom endpoint (bare base_url, not a named custom_providers[] entry) fails its /v1/models probe, the provider group was silently dropped from the model picker entirely. This made the endpoint unusable even when /v1/chat/completions would work fine.
The fix adds an elif branch: if pid == 'custom', a cfg_base_url is configured, but no models were returned by the probe, the group is still added with an empty model list. Users can then select the Custom group and type a model ID manually in the picker's free-form input.
Closes#2542.
