fix: aggregate images by multiscale labels

[Sizerta]

Description

Fixes an AttributeError that occurs when aggregating an image by multiscale labels.

The issue was that multiscale labels are represented as an xarray.DataTree, which doesn’t have a dtype attribute. The fix gets the dtype from the full-resolution pyramid level instead.

Tests

Added a regression test covering single-scale and multiscale combinations of both values and by. The test also checks that the multiscale result matches the corresponding full-resolution result.

• hatch test tests/core/operations/test_aggregations.py — 54 passed, 6 skipped
• hatch check types — passed
• pre-commit — passed locally (check-ast was skipped because Python 3.12 is not installed)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.55%. Comparing base (e0f7d27) to head (ea9069e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1261      +/-   ##
==========================================
+ Coverage   91.40%   91.55%   +0.15%     
==========================================
  Files          53       53              
  Lines        8381     8388       +7     
==========================================
+ Hits         7661     7680      +19     
+ Misses        720      708      -12     

Files with missing lines	Coverage Δ
src/spatialdata/_core/operations/aggregate.py	95.26% <100.00%> (+6.04%)	⬆️

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