feat: add root_dir parameter to oci_layers_on_top

[jonburdo]

This preserves subdirectory structure in layer arcnames if provided.

[jonburdo]

I've just gotten back to testing this with model-registry and verifying that it works as expected there in save_to_oci_registry: kubeflow/model-registry#2521 (comment)

[tarilabs]

I have verified manually with:

poetry run python scripts/test_oras_py.py download quay.io/mmortari/demo20260406:oldsignature

to still be working the old way.

Then, I checked leveraging this new named parameter with:

poetry run python scripts/test_oras_py.py download quay.io/mmortari/demo20260406:newsignature

and local modification as:

diff --git a/scripts/test_oras_py.py b/scripts/test_oras_py.py
index dc5b75a..6968b9f 100644
--- a/scripts/test_oras_py.py
+++ b/scripts/test_oras_py.py
@@ -38,7 +38,7 @@ def main():
             print(f"  modelcard: {modelcard.relative_to(model_dir)}")
         for f in model_files:
             print(f"  {f.relative_to(model_dir)}")
-        oci_layers_on_top(layout_dir, model_files, modelcard)
+        oci_layers_on_top(layout_dir, model_files, modelcard, root_dir="download")
 
         print(f"Pushing to {args.dest} ...")
         oras_py_push(layout_dir, args.dest, insecure=args.insecure)

and results as intended (subdirectory preserved)

Thanks a lot @jonburdo !