Add keep_attrs to discrimination and Contingency

[Kkkakania]

Description

Completes the remaining keep_attrs support requested in #258.

• adds keep_attrs=False to discrimination
• adds keep_attrs=False to Contingency
• preserves observation attributes for DataArray and Dataset outputs when requested
• explicitly clears output attributes when keep_attrs=False, keeping behavior stable across xarray versions
• activates the existing reliability attribute test and covers eager, Dask, DataArray, Dataset, and multidimensional Dataset inputs

Closes #258

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• uv run --extra test pytest -q (2894 passed, 3 skipped)
• pre-commit on all four changed files (all applicable hooks passed)

Checklist (while developing)

• I have added documentation for the new parameters.
• Tests added for pytest.

Pre-Merge Checklist (final steps)

• Based on the current upstream main.
• Contains one focused commit.

[aaronspring]

Thanks. Sorry for not responding

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (e457985) to head (ab3263a).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #456   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files          27       27           
  Lines        2841     2878   +37     
=======================================
+ Hits         2687     2722   +35     
- Misses        154      156    +2     

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