Harden CPS pregnancy take-up rates

[MaxGhenis]

Summary

• use the CPS build year when deriving pregnancy take-up rates
• make the pregnancy-rate helper fall back across nearby CDC and ACS years, matching the ETL CLI behavior
• add a regression test for fallback behavior

Tests

• uv run ruff format policyengine_us_data/db/etl_pregnancy.py policyengine_us_data/datasets/cps/cps.py tests/unit/db/test_etl_pregnancy.py
• uv run ruff check policyengine_us_data/db/etl_pregnancy.py policyengine_us_data/datasets/cps/cps.py tests/unit/db/test_etl_pregnancy.py
• uv run pytest tests/integration/test_cps_generation.py tests/unit/db/test_etl_pregnancy.py -q
• uv run python - <<'PY'
  from policyengine_us_data.db.etl_pregnancy import get_state_pregnancy_rates
  rates = get_state_pregnancy_rates(cdc_year=2024, acs_year=2024)
  print(len(rates))
  print('AL', round(rates['AL'], 6))
  PY