Fix bug unserializable attrs new pyarrow - #1003

The bug was caused by newer versions of pyarrow starting to serialize .attrs (previously they were ignored). Specifically the problem was with transformations objects included in .attrs. These objects aren’t automatically JSON-serializable as they need an explicit conversion like json.dumps(my_transformation.to_ngff(...).to_dict()). When pyarrow attempted to serialize these objects into Parquet, it failed. Since we don’t want these transformation classes in the .parquet file anyway (they belong in Zarr metadata), the fix was to discard .attrs before writing to Parquet so pyarrow wouldn’t try to serialize them.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.11%. Comparing base (28e4975) to head (8228b52).
⚠️ Report is 29 commits behind head on main.

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  Coverage   92.11%   92.11%           
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  Files          48       48           
  Lines        7440     7442    +2     
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+ Hits         6853     6855    +2     
  Misses        587      587