Add modeled Medicaid variables

[hua7450]

Summary

Adds parallel modeled Medicaid variables for API partner household calculations without changing the existing data-backed Medicaid cost path.

• keeps medicaid_cost_if_enrolled and medicaid_cost as data-backed variables for datasets
• adds modeled_medicaid_cost_if_enrolled using the pre-Make Medicaid cost if enrolled data-backed #8499 per-capita Medicaid formula
• adds modeled_medicaid_cost gated by Medicaid enrollment
• adds modeled_medicaid as the API partner-facing modeled Medicaid benefit amount, mirroring medicaid while using the modeled cost path
• updates partner contract fixtures so partners request modeled_medicaid instead of sending medicaid_cost_if_enrolled
• adds focused YAML tests and a changelog fragment

Root cause

PR #8499 made medicaid_cost_if_enrolled data-backed, which is correct for datasets but removed the formula-backed value API partner household calculations need when they do not provide a Medicaid cost input.

Partner contract

API partners should not send medicaid_cost_if_enrolled. When they want PolicyEngine to estimate Medicaid value for household calculations, they should request modeled_medicaid.

Validation

• policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/medicaid/costs -c policyengine_us
• policyengine-core test policyengine_us/tests/policy/baseline/partners/amplifi policyengine_us/tests/policy/baseline/partners/my_friend_ben policyengine_us/tests/policy/baseline/partners/impactica -c policyengine_us
• Static scan confirms partner fixtures no longer contain medicaid_cost_if_enrolled, person-level medicaid, or modeled_medicaid_cost outputs.

Fixes #8533

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ee131a9) to head (7429c36).
⚠️ Report is 43 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8535    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files            9         3     -6     
  Lines          149        32   -117     
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- Hits           149        32   -117     

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

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