fix: zero default scalar for bytes fields in structured dtypes

[safaksm]

Summary

Structured.default_scalar builds each field's zero with np.array([0], dtype)[0], which breaks for bytes fields:

• V (raw bytes): numpy raises TypeError: a bytes-like object is required, not 'int', so create_array fails for any struct with a V field unless fill_value is passed.
• S (byte string): numpy returns b"0", so unwritten chunks read back as "0".

The same types outside a struct already default to zero bytes (S4 to b"", V4 to four zero bytes). This PR uses np.zeros(1, dtype)[0], which gives zero bytes for every fixed-size field.

import numpy as np, zarr

dt = np.dtype([("a", "V4"), ("b", "<f4")])
zarr.create_array(zarr.storage.MemoryStore(), shape=(2,), dtype=dt)  # TypeError

dt = np.dtype([("a", "S4"), ("b", "<f4")])
zarr.create_array(zarr.storage.MemoryStore(), shape=(2,), dtype=dt)[:]  # [(b'0', 0.), (b'0', 0.)]

I hit this while mapping fixed-size HDF5 records (WOD plankton data) to a Zarr struct.

For reviewers

Only new struct arrays created without fill_value change; stored arrays keep theirs.

Author attestation

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

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions (no API change)
• New/modified features documented in docs/user-guide/*.md (no feature change)
• Changes documented as a new file in changes/
• 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 94.69%. Comparing base (246fa20) to head (83ad7e8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4495   +/-   ##
=======================================
  Coverage   94.69%   94.69%           
=======================================
  Files          94       94           
  Lines       13619    13619           
=======================================
  Hits        12897    12897           
  Misses        722      722           

Files with missing lines	Coverage Δ
src/zarr/core/dtype/npy/structured.py	97.14% <100.00%> (ø)

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[d-v-b]

thanks @safaksm, I'll see if we can get this released soon