fix: Error on {Array,Group}::[async_]open[_opt] with additional fields with "must_understand": true

[LDeakin]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 85.98131% with 15 lines in your changes missing coverage. Please review.

Project coverage is 82.23%. Comparing base (a146398) to head (6270292).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
zarrs/src/array/array_async_readable.rs	0.00%	11 Missing ⚠️
zarrs/src/group.rs	90.69%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #149      +/-   ##
==========================================
+ Coverage   82.20%   82.23%   +0.03%     
==========================================
  Files         173      173              
  Lines       24624    24722      +98     
==========================================
+ Hits        20241    20331      +90     
- Misses       4383     4391       +8     

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