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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