fix(video): exclude dropdown separators from model resolution

The registry helpers filtered only the 'None' placeholder, so the eleven
LTX separator rows resolved as models and a POST naming one reached
load_model with a null repo. is_model() now covers both sentinel kinds.

modules/api/video.py

@@ -244,7 +244,7 @@ class APIVideo:
             if engine is not None and family.lower() != engine.lower():
                 continue
             for m in rows:
-                if m.name == 'None':
+                if not models_def.is_model(m):
                     continue
                 items.append(ItemVideoModel(
                     engine=family,