🐛 Fix obstore listdir method using _relativize_path

[antoinegaston]

This PR fixes this issue relativizing paths for ObjectStore.

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)

[codspeed-hq]

Merging this PR will not alter performance

✅ 30 untouched benchmarks

---

_{Comparing antoinegaston:fix-obstore-listdir (ef19e83) with main (e94504e)}

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.86%. Comparing base (4cddcf5) to head (89dbb6e).

Files with missing lines	Patch %	Lines
src/zarr/storage/_obstore.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3657      +/-   ##
==========================================
- Coverage   60.88%   60.86%   -0.03%     
==========================================
  Files          86       86              
  Lines       10182    10184       +2     
==========================================
- Hits         6199     6198       -1     
- Misses       3983     3986       +3     

Files with missing lines	Coverage Δ
src/zarr/testing/store.py	75.00% <100.00%> (ø)
src/zarr/storage/_obstore.py	59.81% <50.00%> (-1.02%)	⬇️

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