fix: write OME channel labels as strings

[stanbot8]

OME-NGFF 0.5 requires a string label for each channel. Write OME channel labels as strings. Update both metadata attributes with 1 attribute call. Closes #1136. All tests pass.

[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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