[DQ Threshold W2.7] `tableCustomSQLQuery`: accept `thresholdUnit`, apply `operator` to a percentage

[TeddyCr]

tableCustomSQLQuery is the one definition that keeps its operator — a user-authored query can legitimately assert in either direction ("must return at least 1 row"). It gains only a unit.

Scope

• ABSOLUTE (default, and today's meaning): evaluate_threshold(threshold, operator, len_rows) — unchanged.
• PERCENTAGE: apply the same operator to len_rows / rowCount * 100 instead of the raw count.
• Denominator must be computed when PERCENTAGE is set, regardless of computePassedFailedRowCount.
• threshold and operator keep their stored values and meanings — no rename, no migration of existing test cases.

Acceptance

• Existing test cases (no thresholdUnit) behave identically
• operator: <= + PERCENTAGE + 1 fails when the query returns more than 1% of table rows
• Every operator behaves correctly against a percentage

Depends on the row-tolerance sub-task.

Design spec: docs/plans/2026-09-11-dq-failure-thresholds.md. Parent epic: #16301.