Model Medicare enrollment as eligible take-up

[MaxGhenis]

Summary

• Model medicare_enrolled as an eligibility-gated take-up variable, matching the Medicaid enrollment pattern.
• Add takes_up_medicare_if_eligible, defaulting to true for eligible people.
• Update Medicare cost, Part D surcharge, and ACA coverage-conflict fixtures to use take-up rather than directly forcing medicare_enrolled.

Root Cause

Previously, medicare_enrolled had defined_for = "is_medicare_eligible" and default_value = True. When a dataset omitted medicare_enrolled, PolicyEngine could treat enrollment as a direct default input instead of a calculated eligibility-and-take-up result, which made downstream datasets vulnerable to overcounting enrollment.

Validation

• git diff --check
• uv run python -m py_compile policyengine_us/variables/gov/hhs/medicare/takes_up_medicare_if_eligible.py
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/hhs/medicare policyengine_us/tests/policy/baseline/gov/aca/eligibility -c policyengine_us (171 passed, 1 warning)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (623a65c) to head (17c2b3b).
⚠️ Report is 156 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8255    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           12         2    -10     
  Lines          173        17   -156     
  Branches         4         0     -4     
==========================================
- Hits           173        17   -156     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[daphnehanse11]

codex and i say looks good!