fix: accept unknown metadata fields with must_understand=false

[NIK-TIGER-BILL]

Summary

Per the Zarr specification, metadata with unknown fields that have must_understand: false should be accepted. Currently, all unknown fields are rejected.

Changes

• Updated metadata parsing to accept unknown fields when must_understand is explicitly set to false.

Testing

• Added unit test verifying that metadata with unknown fields and must_understand: false is accepted.

Fixes #3523

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.54%. Comparing base (b9d3964) to head (f794ca4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4050      +/-   ##
==========================================
+ Coverage   93.53%   93.54%   +0.01%     
==========================================
  Files          88       88              
  Lines       11894    11913      +19     
==========================================
+ Hits        11125    11144      +19     
  Misses        769      769              

Files with missing lines	Coverage Δ
src/zarr/core/group.py	95.30% <100.00%> (+0.09%)	⬆️

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