fix: shift dates for the default model by matching the canonical label

[ardittirana]

Description

method="shift_dates" silently masked dates for the default English model instead of shifting them.

In _redact_entity and the keep_mapping occurrence counter in _build_deidentification_result, a span was treated as a date by comparing the raw entity.entity_type to the literal "DATE". But entity_type holds the model's raw label, and the default model OpenMed-PII-SuperClinical-Small-44M-v1 emits a lowercase date. So date spans never matched and fell through to [DATE] mask placeholders — shift_dates degraded to masking for the default model.

Fix: decide date-ness from the canonical label via a small _is_date_entity helper (entity.canonical_label, falling back to normalize_label(entity.entity_type, lang)), applied at both sites.

Change type

Bug fix (no API change, no new dependency).

Tests run

• pytest tests/unit/test_pii.py — 113 passed (adds test_redact_shift_dates_uses_canonical_label, which fails on the base branch and passes here).
• ruff check / ruff format --check — clean.

Docs / changelog

Added an entry under [Unreleased] > Fixed in CHANGELOG.md.

Linked issue

Fixes #513 (addresses #408).

[maziyarpanahi]

Thank you @ardittirana. I reviewed this against #513 / OM-323 and added one maintainer follow-up commit: test: cover canonical date shifting labels.

What I changed:

• extended _is_date_entity() so DATE_OF_BIRTH is treated as a shiftable date label, not just DATE;
• added end-to-end mocked deidentify() regressions for the default English model's lowercase date label;
• added coverage for raw date_of_birth labels;
• added keep_mapping=True coverage to ensure shifted dates are not counted or suffixed like mask placeholders.

Verification on the current PR checkout:

• PYTHONPATH=/private/tmp/openmed-pr-559 /Users/maziyar/Developer/openmed/.venv/bin/python -m pytest tests/unit/test_pii.py -q -> 116 passed
• /Users/maziyar/Developer/openmed/.venv/bin/ruff check openmed/core/pii.py tests/unit/test_pii.py CHANGELOG.md -> passed
• /Users/maziyar/Developer/openmed/.venv/bin/ruff format --check openmed/core/pii.py tests/unit/test_pii.py -> passed

I also copied the labels from #513 onto the PR. The branch is mergeable with no conflicts; GitHub has not attached hosted checks to the new head commit yet, so I verified the touched behavior locally.