Impute unavailable prior-year CPS income

[MaxGhenis]

Summary

• replace prior-year CPS income sentinel outputs with current-year income fallbacks
• keep previous_year_income_available false when prior-year values are missing or sentinel-coded
• add coverage for missing previous-year matches, -1, and -9999 prior-year income sentinels

Rationale

The CPS prior-year join previously encoded unavailable prior-year income as -1 via fillna(-1). In the live Enhanced CPS this made self_employment_income_last_year equal -1 for most person records, which reads as a real dollar amount downstream. This PR keeps numeric income fields numeric and interpretable while preserving the availability flag.

Tests

• uv run pytest tests/unit/datasets/test_cps_file_handles.py tests/unit/datasets/test_cps_identification.py tests/unit/test_weeks_unemployed.py
• uv run ruff check policyengine_us_data/datasets/cps/cps.py tests/unit/datasets/test_cps_file_handles.py