fix: list nested prefixes when FsspecStore path is "" or "/"

[bendichter]

list_dir and list_prefix joined the store path and the prefix with a "/", so a store rooted at "" or "/" asked the filesystem for "/a" or "//a". ReferenceFileSystem keys entries by bare names and does not find those, so nested groups listed as empty. FsspecStore.from_url with "reference://" gives a path of "", so this affected kerchunk-style references opened the usual way. Reads were already handled by _dereference_path.

import json

import zarr

group = json.dumps({"zarr_format": 3, "node_type": "group"})
refs = {"version": 1, "refs": {"zarr.json": group, "a/zarr.json": group, "a/b/zarr.json": group}}

root = zarr.open_group("reference://", storage_options={"fo": refs}, mode="r")

print(list(root.group_keys()))       # ['a']
print(list(root["a"].group_keys()))  # expected ['b']
print(root["a/b"].path)              # 'a/b', so the group exists

With zarr 3.4.0 and fsspec 2026.9.0, the output is:

['a']
[]
a/b

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)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.44%. Comparing base (319bfa4) to head (1ed2d45).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4452      +/-   ##
==========================================
+ Coverage   94.43%   94.44%   +0.01%     
==========================================
  Files          93       93              
  Lines       13219    13221       +2     
==========================================
+ Hits        12483    12487       +4     
+ Misses        736      734       -2     

Files with missing lines	Coverage Δ
src/zarr/storage/_fsspec.py	91.80% <100.00%> (+0.09%)	⬆️

... and 1 file with indirect coverage changes

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[d-v-b]

thanks for this, and sorry for the bugged behavior. I'm having a look now