Fix Pension Credit savings credit formula

[MaxGhenis]

Fixes #1753.

Summary

• annualize the Savings Credit threshold in is_savings_credit_eligible
• require qualifying income to exceed, not merely equal, the threshold
• compute the maximum Savings Credit from standard_minimum_guarantee, not the full minimum_guarantee with additions
• compute the amount-B reduction from pension_credit_income, while keeping savings_credit_income as qualifying income for amount A
• add focused YAML tests for the annual threshold, standard-minimum cap, and amount-B income source

Validation

• uv run ruff format --check policyengine_uk/variables/gov/dwp/pension_credit/savings_credit/is_savings_credit_eligible.py policyengine_uk/variables/gov/dwp/pension_credit/savings_credit/savings_credit.py
• uv run ruff check policyengine_uk/variables/gov/dwp/pension_credit/savings_credit/is_savings_credit_eligible.py policyengine_uk/variables/gov/dwp/pension_credit/savings_credit/savings_credit.py
• uv run policyengine-core test policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/savings_credit/is_savings_credit_eligible.yaml policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/savings_credit/savings_credit.yaml -c policyengine_uk -> 7 passed

This was identified by the Axiom UK State Pension Credit section 3 oracle comparison: TheAxiomFoundation/axiom-encode#533 and TheAxiomFoundation/rulespec-uk#11.