Summary

Fixes #4032.

ObjectStore.list_dir can receive a directory-marker entry that matches the requested non-root prefix itself, such as path "g" for prefix "g". That entry represents the prefix, not a child of the prefix, so _transform_list_dir now skips exact prefix-marker entries before relativizing returned paths.

This also avoids yielding empty relative paths and adds regression coverage for both "g" and "g/" prefix inputs.

For reviewers

Please check whether handling the exact-prefix marker in _transform_list_dir is the right level. I kept the change local to ObjectStore because this marker entry is an object-store delimiter-listing artifact.

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

Validation

• python3 ci/check_changelog_entries.py
• git diff --check
• python3 -m py_compile src/zarr/storage/_obstore.py tests/test_store/test_object.py

I could not run the targeted pytest locally because this Python environment is missing the project test dependency stack (pytest, obstore, numcodecs, donfig).