fix: accept lists of byte values as fill values for byte dtypes

[barlowa124]

Summary

Refs #4453, fourth checklist item. A fill_value on a byte data type
must be written as a base64 string today, though the Zarr V3 spec also
permits a list of byte values. from_json_scalar on
NullTerminatedBytes, RawBytes and VariableLengthBytes rejected
that form, so a spec-valid fill_value: [65] raised TypeError.

The new check_json_byte_list in npy/common.py matches a JSON list
of integers 0 through 255, booleans excluded. All three byte dtypes
accept the form and convert through bytes(...). Anything else falls
through to the existing TypeError, whose message now names both
accepted forms.

For reviewers

Open question: should a byte list longer than the fixed length be
rejected rather than truncated by np.bytes_/np.void? Silent
truncation is kept because the base64 path already behaves that way,
keeping the two forms consistent.

Author attestation

• I am a human. These are my changes. I have reviewed and understood every change and can explain why each is correct.

TODO

Done:

• Add unit tests and/or doctests in docstrings
• Changes documented as a new file in changes/

Outstanding:

• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.70%. Comparing base (069fd20) to head (4d09a43).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4493   +/-   ##
=======================================
  Coverage   94.69%   94.70%           
=======================================
  Files          94       94           
  Lines       13606    13627   +21     
=======================================
+ Hits        12884    12905   +21     
  Misses        722      722           

Files with missing lines	Coverage Δ
src/zarr/core/dtype/npy/bytes.py	99.51% <100.00%> (+0.01%)	⬆️
src/zarr/core/dtype/npy/common.py	99.20% <100.00%> (+0.01%)	⬆️

... and 1 file with indirect coverage changes

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