fix: auto-chunking when auto-sharding 1MiB number

[ilan-gold]

Fixes #3602

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)

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 59.14%. Comparing base (20670e6) to head (2286a2d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/zarr/core/chunk_grids.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3603      +/-   ##
==========================================
- Coverage   59.15%   59.14%   -0.02%     
==========================================
  Files          86       86              
  Lines       10170    10172       +2     
==========================================
  Hits         6016     6016              
- Misses       4154     4156       +2     

Files with missing lines	Coverage Δ
src/zarr/core/chunk_grids.py	62.29% <66.66%> (-1.04%)	⬇️

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[ilan-gold]

Having this would have caught the issue, so I figured I'd add it in