test(video routes): accept vae_quant in the fake backend stub

The video route forwards vae_quant to validate_load_request, but the
_FakeBackend stub in test_video_routes.py had not been updated to accept
it, so all 11 route tests raised TypeError: unexpected keyword argument
'vae_quant'. Add the keyword to mirror the real backend signature.

studio/backend/tests/test_video_routes.py

@@ -81,6 +81,7 @@ class _FakeBackend:
         model_kind = None,
         transformer_quant = None,
         text_encoder_quant = None,
+        vae_quant = None,
     ):
         # Mirror the real backend's cheap validation so the route's
         # validate-before-evict ordering is exercised.