Require pure Medicare enrollment input support

[MaxGhenis]

Summary

• add a PolicyEngine variable helper that distinguishes pure input variables from conditional/formula variables
• make CPS Medicare enrollment export depend on medicare_enrolled being a pure input, not merely present in policyengine-us
• add unit coverage for rejecting formula, conditional, and non-input variables

Context

This pairs with the policyengine-us cleanup that makes medicare_enrolled a pure data-populated input. Until that model change is installed, this avoids treating a conditionally-defined variable as a storable leaf input.

Testing

• uv run ruff format policyengine_us_data/utils/policyengine.py tests/unit/test_medicare_part_b_inputs.py
• uv run ruff check policyengine_us_data/utils/policyengine.py tests/unit/test_medicare_part_b_inputs.py
• uv run pytest tests/unit/test_medicare_part_b_inputs.py

[MaxGhenis]

Closing and recreating from a PolicyEngine org branch so repository CI can run; fork PRs are rejected by check-fork.