Fix ZarrParser crash on S3 directory-marker objects

[TomNicholas]

Summary

Fixes #1052.

obstore strips the trailing slash from listed S3 object keys, so build_1d_chunk_mapping's endswith("/") check can never filter out zero-byte "directory marker" objects (created by e.g. aws s3 sync, s3fs, or boto3.put_object(Key=prefix + "/")) out of the discovered chunk keys. The marker then fails int() parsing in build_chunk_manifest, crashing with e.g. ValueError: invalid literal for int() with base 10: 'precip'.

This isn't a rare edge case — of 152 real-world public Zarr v2 stores I swept (NOAA/NCAR/weatherbench2 datasets on AWS Open Data), 26 failed with this exact error.

Fix

Filter on whether each listed key actually starts with the full array_chunks_prefix instead of relying on the (now ineffective) trailing-slash check. A directory marker is keyed exactly as the prefix with its trailing slash removed, so it's one character shorter than the prefix and can never satisfy startswith(prefix) — this holds regardless of whether obstore has already stripped the marker's own trailing slash, so it's robust to the underlying cause rather than papering over one specific symptom.

Test plan

• Added test_build_chunk_manifest_ignores_directory_marker_object in virtualizarr/tests/test_parsers/test_zarr.py. Since no in-memory or local obstore backend reproduces the S3-specific slash-stripping quirk itself (writes and lists are both path-normalized consistently for those backends, so a marker written via MemoryStore.put_async never even collides with a nested-chunk listing), the test fabricates the exact listing shape a real S3 store returns and feeds it to build_chunk_manifest directly.
• Confirmed the new test fails with the reported ValueError against the pre-fix code, and passes after the fix.
• Full existing test suite passes locally (pytest virtualizarr/tests), ruff check/ruff format clean, mypy clean on the changed file.
• Also verified the fix against the real public bucket from the original bug report (s3://noaa-nwm-retrospective-2-1-zarr-pds/precip.zarr), which now opens successfully instead of raising.

[This is Claude Code on behalf of TomNicholas]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.39%. Comparing base (117acc7) to head (5cc3370).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1055   +/-   ##
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  Coverage   90.39%   90.39%           
=======================================
  Files          37       37           
  Lines        2248     2249    +1     
=======================================
+ Hits         2032     2033    +1     
  Misses        216      216           

Files with missing lines	Coverage Δ
virtualizarr/parsers/zarr.py	97.41% <100.00%> (+0.01%)	⬆️

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