feat: support for relative (percent) metrics in data contract checks.

[HenrikSpiegel]

• Tests pass (uv run pytest)
• Code formatted (uv run ruff check --fix && uv run ruff format)
• README.md updated (if relevant)
• CHANGELOG.md entry added

#1228

[jschoedl]

Thank you for the PR! A few notes, and please add test cases for this in the tests directory - e.g.:

• test that percent really appends %
• test that no % exists for the default case or rows
• test that e.g. check_property_null_values with percent produces the correct sodacl check
• test that a percent sign in check_row_count behaves correctly

[jochenchrist]

Thanks for this, and apologies it sat open long enough to be overtaken by events.

This is now superseded by the v1.0.0 refactor in #1279 ("Replace Soda Core with ibis test engine"), which landed the same feature through a different path:

• The target file is gone. datacontract/engines/data_contract_checks.py was removed in Replace soda with ibis #1279; the check-building code moved to datacontract/export/sodacl_check_builder.py. This PR no longer applies cleanly.
• Percent thresholds are now supported natively. datacontract test honors ODCS quality.unit: percent on the count-of-bad-rows metrics (nullValues, missingValues, invalidValues), comparing the threshold against the failed fraction of the row count. Percent on metrics where a row fraction has no meaning (rowCount, duplicateValues) logs a warning and falls back to the absolute count, matching the behavior in this PR. See is_percent_unit() in datacontract/engines/checks/create_checks.py, threshold_is_percent in check_spec.py, the comparison logic in ibis/ibis_check_execute.py, and tests/test_test_quality_percent_severity.py.

That resolves #1228, so I'm closing this as already implemented.

One thing not covered by #1279: the SodaCL export (sodacl_check_builder.py) still emits count-only metrics and doesn't apply a % suffix. If emitting percent thresholds in the SodaCL export is still useful to you, a smaller follow-up scoped to that file would be welcome.