Source local LA uprating factors from policyengine-uk

[MaxGhenis]

Summary

• read the LA ONS income uprating assumptions from policyengine-uk when that parameter subtree is available
• replace the direct use of raw module constants in the LA loss builder with a helper call
• keep a compatibility fallback for the current released policyengine-uk dependency so this PR can merge before the next policyengine-uk release
• add focused tests for the parameter-backed path, fallback path, and missing-year failure

Testing

• uv run pytest -q policyengine_uk_data/tests/test_local_la_extras.py
• uv run python - <<"PY"
  from policyengine_uk_data.datasets.local_areas.local_authorities.loss import create_local_authority_target_matrix
  print(create_local_authority_target_matrix.name)
  PY
• uvx ruff check policyengine_uk_data/targets/sources/local_la_extras.py policyengine_uk_data/datasets/local_areas/local_authorities/loss.py policyengine_uk_data/tests/test_local_la_extras.py