Fix _maybe_auto_chunk to not rechunk dask arrays

[aaronspring]

Avoid xarray bug with partial chunk dicts like {'iteration': 'auto'} by only chunking non-dask arrays and object-dtype variables.

non breaking change

should fix src/climpred/tests/test_bootstrap.py::test_bootstrap_PM_lazy_result

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.49%. Comparing base (3fd4292) to head (9a073de).
⚠️ Report is 1 commits behind head on fix-xarray-v2025.

Files with missing lines	Patch %	Lines
src/climpred/bootstrap.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                  @@
##           fix-xarray-v2025     #884      +/-   ##
====================================================
- Coverage             91.52%   91.49%   -0.03%     
====================================================
  Files                    59       59              
  Lines                  6266     6270       +4     
====================================================
+ Hits                   5735     5737       +2     
- Misses                  531      533       +2     

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