tests/_headless.py resolves the ComfyUI tree from COMFYUI_ROOT and the weight
directory from KEEP_MODEL_DIR (no hardcoded paths, actionable errors), loads
extra_model_paths.yaml through utils.extra_config like the server does,
registers a package under an importable alias with __path__ set and can stub
only the ui preview helpers.
tests/test_headless_bootstrap.py proves the helper on a dummy package: alias
resolution, submodule import, the missing-environment-variable errors and the
absence of machine-specific paths.
