Add local-authority target uprating parameters

[MaxGhenis]

Summary

• add named policyengine-uk economic-assumption parameters for the ONS LA income target uprating factors
• keep the values explicit and self-documenting instead of leaving them as raw numbers in policyengine-uk-data
• cover the new parameter paths with a focused regression test

Testing

• uv run pytest -q policyengine_uk/tests/test_local_authority_target_parameters.py
• uvx ruff check policyengine_uk/tests/test_local_authority_target_parameters.py