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.