Models: avoid hidden models in default fallback

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

routes/model_routes.py

@@ -479,6 +479,15 @@ def _model_endpoint_error_message(base_url: str, ping: Dict[str, Any] = None) ->
     return "No models found for that provider/key."
 
 
+def _visible_models(cached_models, hidden_models):
+    """Filter cached model IDs by hidden_models. Returns list of visible IDs."""
+    all_models = json.loads(cached_models) if isinstance(cached_models, str) else (cached_models or [])
+    if not hidden_models:
+        return all_models
+    hidden = set(json.loads(hidden_models) if isinstance(hidden_models, str) else (hidden_models or []))
+    return [m for m in all_models if m not in hidden]
+
+
 def setup_model_routes(model_discovery):
     router = APIRouter(prefix="/api")
 
@@ -1331,9 +1340,9 @@ def setup_model_routes(model_discovery):
             chat_url = build_chat_url(base)
             if not model and getattr(ep, "cached_models", None):
                 try:
-                    models = _json.loads(ep.cached_models) if isinstance(ep.cached_models, str) else ep.cached_models
-                    if models:
-                        model = models[0]
+                    visible = _visible_models(ep.cached_models, getattr(ep, "hidden_models", None))
+                    if visible:
+                        model = visible[0]
                 except Exception:
                     pass
             return {"endpoint_id": ep.id, "endpoint_url": chat_url, "model": model}