Fix save/load for scalar arrays

[gaoflow]

Fixes #3469.

save_array() and load() used slice(None) as the full-array selection. That works for arrays with at least one dimension, but for 0-dimensional arrays it is interpreted as one index and raises IndexError: too many indices for array; expected 0, got 1.

This changes both paths to use Ellipsis, which still means full-array selection for N-dimensional arrays and normalizes to an empty selection for scalar arrays. The regression test covers Group.__setitem__ with both an integer scalar array and a bytes scalar array, then verifies direct indexing and zarr.load().

TODO:

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

Local verification:

• uv run --frozen pytest tests/test_api.py::test_group_setitem_loads_scalar_arrays -q
• uv run --frozen pytest tests/test_api.py::test_save tests/test_api.py::test_load_array tests/test_api.py::test_load_local tests/test_api.py::test_load_zip -q
• uv run --frozen ruff check src/zarr/api/asynchronous.py tests/test_api.py
• uv run --frozen ruff format --check src/zarr/api/asynchronous.py tests/test_api.py
• uv run --frozen python -m compileall -q src/zarr/api/asynchronous.py tests/test_api.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.50%. Comparing base (036ede7) to head (5d4f445).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4103   +/-   ##
=======================================
  Coverage   93.50%   93.50%           
=======================================
  Files          90       90           
  Lines       11981    11981           
=======================================
  Hits        11203    11203           
  Misses        778      778           

Files with missing lines	Coverage Δ
src/zarr/api/asynchronous.py	94.05% <100.00%> (ø)

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[d-v-b]

thanks!