test: cover data-class non-detections

[slegarraga]

Closes #405

What

• Adds regression coverage for the data-class rule's documented OTP limit: a six-digit code bound for an external destination stays PASS.
• Adds coverage for an undashed SSA-valid SSN staying PASS, using the same digits as the valid dashed test value to prove detection depends on shape.
• Groups both cases under a documented non-detections section beside the existing precision tests.

Verification

• .venv/bin/pytest tests/unit/test_rule_data_classes.py -q — 16 passed.
• .venv/bin/ruff check tests/unit/test_rule_data_classes.py — passed.
• .venv/bin/ruff format --check tests/unit/test_rule_data_classes.py — passed.
• git diff --check — clean.