fix: support zarr3 format

[parallelArchitect]

The COSEM API now returns format: "zarr3" for some images, which
the pydantic ImageFormat literal rejected as invalid. Hit this
while pulling jrc_hela-3 / er-mem_pred to validate #124 against
real data — unrelated to that fix, splitting out separately.

[tlambert03]

thanks @parallelArchitect! and thanks for #140 as well... I'll get to that next.
I pushed a couple additional changes to this branch to actually make the full read-path work for zarr3 data, and added a test. Will fix the pre-commit issue elsewhere

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.64%. Comparing base (8b0126b) to head (64de69d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
+ Coverage   83.57%   83.64%   +0.06%     
==========================================
  Files          47       47              
  Lines        3026     3032       +6     
==========================================
+ Hits         2529     2536       +7     
+ Misses        497      496       -1     

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