fix: surface invalid encounter timezone errors

[Utkarsh3725]

What & why

Removes the unreachable tz_localize fallback in EncounterRecencyTransformer._parse_dates.

When timezone is set, pd.to_datetime(..., utc=True) already returns timezone-aware parsed dates, so tz_convert() is the correct path. The old broad except Exception fallback only made invalid
timezone errors worse by replacing the useful timezone error with a misleading "Already tz-aware" error.

Closes #201.

Checklist

• make ci passes locally (lint -> collection -> tests -> coverage >= 92%)
• New/changed public API has docstrings and is exported in the subpackage __init__.py
• Tests added or updated
• CHANGELOG.md updated under [Unreleased]
• Added yourself to CONTRIBUTORS.md

Testing

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 .venv/bin/python -m pytest tests/test_encounter_timezone.py tests/test_preprocessing.py -q
  • Result: 68 passed
• .venv/bin/python -m flake8 philanthropy/preprocessing/_encounter_recency.py

I could not run the coverage version locally because pytest coverage hit a local NumPy import-loader issue in this environment before collecting tests. The focused behavior tests pass.