chore: use coverage run instead of pytest --cov

[maxrjones]

The pytest11 entry point (zarr = "zarr.testing") imports 76 zarr modules
during plugin loading, before pytest-cov starts measuring. This caused all
module-level code (class definitions, imports, decorators) to appear uncovered,
dropping reported coverage well below actual test coverage.

coverage run -m pytest starts tracing before any imports, resolving the issue.

Increases the codecov reported coverage from ~60% to >90%.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.86%. Comparing base (2dc4036) to head (50a4db5).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3847       +/-   ##
===========================================
+ Coverage   62.13%   92.86%   +30.73%     
===========================================
  Files          86       86               
  Lines       10630    10630               
===========================================
+ Hits         6605     9872     +3267     
+ Misses       4025      758     -3267     

see 74 files with indirect coverage changes

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