Add a no-economic-assumptions scenario

[MaxGhenis]

Summary

• add a reusable no_economic_assumptions scenario that zeroes forecast growth rates from the model input year onward before data load
• reset already-uprated multi-year datasets when that scenario is used so the data path and parameter path stay aligned
• document the scenario and add regression coverage for both parameter/input uprating and multi-year dataset handling

Closes #1253.

Testing

• uv run python -m pytest -q policyengine_uk/tests/test_no_economic_assumptions.py
• uv run python -m pytest -q policyengine_uk/tests
• uvx ruff check policyengine_uk/scenarios/no_economic_assumptions.py policyengine_uk/scenarios/__init__.py policyengine_uk/simulation.py policyengine_uk/tests/test_no_economic_assumptions.py
• uvx ruff format --check policyengine_uk/scenarios/no_economic_assumptions.py policyengine_uk/scenarios/__init__.py policyengine_uk/simulation.py policyengine_uk/tests/test_no_economic_assumptions.py

Notes

• make documentation currently fails locally because the installed jupyter entrypoint points at a missing Homebrew Python 3.9 interpreter (/opt/homebrew/opt/[email protected]/bin/python3.9).