fix(array): keep other fields intact when writing one field of a structured array

[d-v-b]

🤖 AI text below 🤖

Writing one field of a structured array (fields="b") no longer overwrites the other fields of each selected record. Passing a tuple that names several fields to a write now raises IndexError, as a list already did.

On main, _set_selection validates fields and then drops it, casting the value to the full structured dtype. For example, z.set_basic_selection(slice(None), np.full(10, 7.0), fields="b") sets every field of every record to 7. This happens for v2, v3, and sharded v3 arrays, and for basic, orthogonal, coordinate, mask, and block writes.

With this change, a field write reads the selected records into a full-dtype buffer, assigns the field, and writes the whole records back. Missing chunks are read as the fill value even when read_missing_chunks is false, as they are for any partial chunk write. The cost is one extra read of the selection, and only field writes pay it. Like any partial-chunk write, the read and the write are not atomic.

Tests:

• test_set_selections_with_fields was marked xfail with the reason "fields are not supported in v3". It now runs, filtering only the UnstableSpecificationWarning for its S3 field. Its multi-field cases passed a 3-element value to a 2-element selection, so a shape check fired before the multi-field check; they now pass matching values.
• New test_set_field_preserves_other_fields covers v2, v3, and sharded v3, with read_missing_chunks true and false, using every selection type, including a partly written chunk and a missing shard. It fails on main and passes here.
• New test_set_multiple_fields_as_tuple_raises checks the tuple error case.

Context

The agent found this bug while reviewing #3996, which fixes the read side (fields= reads raise TypeError on v2 and v3). This PR fixes writes only and does not depend on #3996. test_get_selections_with_fields stays xfailed until the read fix lands. This PR replaces the agent's fork draft d-v-b#400; both use the same branch.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.76%. Comparing base (bd0dc84) to head (4423e60).

Files with missing lines	Patch %	Lines
src/zarr/core/array.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4503      +/-   ##
==========================================
+ Coverage   94.71%   94.76%   +0.05%     
==========================================
  Files          94       94              
  Lines       13663    13668       +5     
==========================================
+ Hits        12941    12953      +12     
+ Misses        722      715       -7     

Files with missing lines	Coverage Δ
src/zarr/core/indexing.py	97.37% <100.00%> (+0.79%)	⬆️
src/zarr/core/array.py	98.44% <90.90%> (+<0.01%)	⬆️

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