Fix state pension relative reform path

[MaxGhenis]

Summary

• read state pension contribution reforms from the active parameter tree instead of the period snapshot that was ignoring
• apply the relative state pension reform multiplier to the aggregated state pension amount
• add a microsimulation regression test and changelog entry

Testing

• .. [100%]
  =============================== warnings summary ===============================
  ../policyengine-uk-fix-private-default/.venv/lib/python3.13/site-packages/pydantic/_internal/_config.py:323
  /Users/maxghenis/worktrees/policyengine-uk-fix-private-default/.venv/lib/python3.13/site-packages/pydantic/_internal/_config.py:323: PydanticDeprecatedSince20: Support for class-based config is deprecated, use ConfigDict instead. Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.11/migration/
  warnings.warn(DEPRECATION_MESSAGE, DeprecationWarning)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
2 passed, 1 warning in 12.51s

Closes #1178.