Add has_blackwell_gpu to the mlx worker test's wheel_utils stub (#6980)

worker.py imports has_blackwell_gpu from utils.wheel_utils, but _load_worker_module
stubs utils.wheel_utils with a fixed name tuple that omitted it, so loading the worker
raised ImportError (cannot import name 'has_blackwell_gpu') and Backend CI could not
collect test_mlx_training_worker_config.py. Add the name to the stub so it matches
worker.py's imports.

studio/backend/tests/test_mlx_training_worker_config.py

@@ -37,6 +37,7 @@ def _load_worker_module():
         for name in (
             "direct_wheel_url",
             "flash_attn_wheel_url",
+            "has_blackwell_gpu",
             "install_wheel",
             "probe_torch_wheel_env",
             "url_exists",