Make Medicaid cost if enrolled data-backed

[MaxGhenis]

Summary

• remove the PolicyEngine-US formula for medicaid_cost_if_enrolled so datasets provide the conditional Medicaid cost
• keep medicaid_cost zero for non-enrollees and using the data input for enrollees
• compute Medicaid federal share from the legal Medicaid category, so expansion FMAP does not depend on any cost allocation group

Tests

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/hhs/medicaid/costs -c policyengine_us

Note: I also started the broader policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/hhs --mode per-subdir; it reached the Medicare subdirectory with no failure output but did not produce progress, so I stopped it and kept the targeted Medicaid suite as the local gate.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.06%. Comparing base (92fdab8) to head (1da04a6).
⚠️ Report is 100 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##              main    #8499       +/-   ##
============================================
- Coverage   100.00%   44.06%   -55.94%     
============================================
  Files            2        3        +1     
  Lines           57       59        +2     
  Branches         1        7        +6     
============================================
- Hits            57       26       -31     
- Misses           0       33       +33     

Flag	Coverage Δ
unittests	44.06% <100.00%> (-55.94%)	⬇️

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