Guard income imputation rescale against zero-total baseline

[MaxGhenis]

Summary

impute_over_incomes in policyengine_uk_data/datasets/imputations/income.py computed new_income_total / original_income_total with no guard for the case where original_income_total == 0. That case is reachable from within impute_income itself, where the zero_weight_copy dataset is constructed specifically so its imputation columns contribute nothing — and it will become more common as additional callers of impute_over_incomes land.

Before: ZeroDivisionError (or silent NaN/inf propagating into household rent and mortgage columns).

After: _safe_rescale_factor(original, new) returns 1.0 when the baseline is zero (leaves rent/mortgage untouched), otherwise returns new / original as before.

Test plan

• uv run pytest policyengine_uk_data/tests/test_income_rescale_factor.py -q passes.
• Tests cover the zero-original case, the normal non-zero case, and finiteness of the returned factor across several representative inputs.

Finding U3 from the bug hunt.

[MaxGhenis]

Self-review: APPROVE. See /tmp/bug-hunt/reviews/uk-data-reviews.md for detail. (GitHub blocks self-approve, so leaving as comment.)