Test lazy execution across deterministic metrics

[Kkkakania]

Closes #250.

This adds a parameterized regression test covering every deterministic metric in the skip-NaN suite. The test installs a scheduler that raises on any Dask execution, calls each metric with chunked inputs, and verifies the result remains a Dask collection.

Validation:

• pytest -q xskillscore/tests/test_skipna_functionality.py (64 passed)
• pre-commit run --files xskillscore/tests/test_skipna_functionality.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.52%. Comparing base (e457985) to head (4dbdc3e).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
- Coverage   94.57%   94.52%   -0.06%     
==========================================
  Files          27       27              
  Lines        2841     2847       +6     
==========================================
+ Hits         2687     2691       +4     
- Misses        154      156       +2     

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