fix: write OME channel labels as strings

[stanbot8]

Change

The OME-NGFF 0.5 channel model requires each channel label to be a string. SpatialData wrote numeric channel coordinates without conversion. This produced the invalid metadata reported in issue 1136.

This change writes OME channel labels as strings. It stores the SpatialData channel coordinates separately. This preserves integer coordinates and numeric-looking string coordinates without a type guess. It publishes both metadata representations with 1 atomic attribute update. Old stores continue to use their OME labels.

Closes #1136.

Commands

uvx pre-commit run --files src/spatialdata/_io/_utils.py src/spatialdata/_io/io_raster.py tests/io/test_metadata.py

pytest tests/io/test_metadata.py -q

pytest --cov --color=no -n auto --dist worksteal -q

All commands pass. The full suite reports 1366 passed, 7 skipped, and 1 expected failure.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.45%. Comparing base (eb4fb3d) to head (0257c80).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1168   +/-   ##
=======================================
  Coverage   92.44%   92.45%           
=======================================
  Files          51       51           
  Lines        7820     7828    +8     
=======================================
+ Hits         7229     7237    +8     
  Misses        591      591           

Files with missing lines	Coverage Δ
src/spatialdata/_io/_utils.py	86.92% <100.00%> (+0.25%)	⬆️
src/spatialdata/_io/io_raster.py	89.20% <100.00%> (+0.15%)	⬆️

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