Fix state pension reforms having no budget impact

[MaxGhenis]

Summary

• switch state pension formulas to leaf-level parameter access so simulation-year reforms are visible
• preserve the cap/share logic for basic state pension and scale additional pension consistently
• add a focused microsimulation regression test

Context

• supersedes the stale conflicting branch in Fix state pension reforms having no budget impact (#1178) #1509 with a fresh re-cut on current main
• intentionally leaves out the stale reforms_config.yaml edits from the old PR

Validation

• uv run pytest policyengine_uk/tests/microsimulation/test_state_pension_reform.py -q