Add 'custom' to the provider order so custom OpenAI-compatible
endpoints appear in /model list. Probes the endpoint's /models API
to dynamically discover available models.
Changes:
- Add 'custom' to _PROVIDER_ORDER in list_available_providers()
- Add _get_custom_base_url() helper to read model.base_url from config
- Add custom branch in provider_model_ids() using fetch_api_models()
- Custom endpoint detection via base_url presence for has_creds check
Based on PR #1612 by @aashizpoudel.
Co-authored-by: Aashish Poudel <aashizpoudel@users.noreply.github.com>