Both get_default_chat and _recover_empty_session_model picked the
first model from cached_models[0] without checking hidden_models.
If the first cached model was hidden (e.g. minimax-m3), it was
returned as the default or used to repair empty session models,
even though the model list endpoints already filter hidden_models.
- Add _visible_models() helper that filters cached_models by
hidden_models (mirrors the filtering in list_model_endpoints)
- Use _visible_models() in get_default_chat fallback (when no
explicit default_model is saved)
- Use _visible_models() in _recover_empty_session_model (when
repairing a session whose model field is empty before chat send)
- Add regression tests for hidden-model filtering in default chat
resolution, and unit tests for _visible_models helper
7cc8fdb2f5