feat: align Zarr array metadata class names

[victorwon2001]

Summary

Rename the concrete Zarr array metadata dataclasses to ArrayMetadataV2 and ArrayMetadataV3, while keeping ArrayV2Metadata and ArrayV3Metadata as aliases to the same classes. Both spellings are available from their versioned modules and zarr.core.metadata; the ArrayMetadata union uses the canonical names without changing metadata serialization, equality or array behavior.

For reviewers

Existing imports, isinstance checks, class patterns and historical pickle class references continue to resolve through the aliases, but reflection such as __name__ now reports the new canonical names. Focused and broader metadata/array/group regressions, strict typing, scoped hooks, changelog validation and documentation checks passed; a PR-numbered feature changelog and human attestation remain publication steps.

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
• 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)

Fixes #3843

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.73%. Comparing base (bd0dc84) to head (23c4aa0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4527      +/-   ##
==========================================
+ Coverage   94.71%   94.73%   +0.01%     
==========================================
  Files          94       94              
  Lines       13663    13665       +2     
==========================================
+ Hits        12941    12945       +4     
+ Misses        722      720       -2     

Files with missing lines	Coverage Δ
src/zarr/core/metadata/__init__.py	100.00% <100.00%> (ø)
src/zarr/core/metadata/v2.py	91.25% <100.00%> (+0.59%)	⬆️
src/zarr/core/metadata/v3.py	97.19% <100.00%> (+0.28%)	⬆️

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