Add AOTC student eligibility inputs

[MaxGhenis]

Summary

• add lower-level American Opportunity Credit eligibility inputs for the statutory student, documentation, institution, identification, residency, improper-claim, and filing-status predicates
• compute is_eligible_for_american_opportunity_credit from those predicates instead of leaving it as an unpopulated input
• model the section 25A(g)(1) identification transition: TINs through 2025, qualifying SSNs for tax years beginning after December 31, 2025
• gate Form 1098-T/payee-statement and institution-EIN requirements with time-varying AOTC eligibility parameters instead of hardcoded formula years
• update AOTC variable and parameter references to statute sources

Primary sources

• 26 U.S.C. § 25A for AOTC amount, eligible-student limits, first-four-years, prior claim years, felony drug conviction, eligible institution, identification, 1098-T/payee statement, institution EIN, nonresident-alien, improper-claim, MFS, and refundability rules
• 20 U.S.C. § 1091(a)(1), incorporated by 26 U.S.C. § 25A(b)(3)(A), for the recognized-credential enrollment rule
• 26 U.S.C. § 24(h)(7), incorporated by 26 U.S.C. § 25A(g)(1)(C) for years beginning after December 31, 2025, for the qualifying-SSN definition
• 26 U.S.C. § 6013(g)-(h), referenced by 26 U.S.C. § 25A(g)(7), for resident treatment elections relevant to the nonresident-alien disallowance

Tests

• uv run ruff format policyengine_us/variables/gov/irs/credits/education/american_opportunity_credit policyengine_us/parameters/gov/irs/credits/education/american_opportunity_credit policyengine_us/tests/policy/baseline/gov/irs/credits/education/american_opportunity_credit && uv run ruff check policyengine_us/variables/gov/irs/credits/education/american_opportunity_credit policyengine_us/parameters/gov/irs/credits/education/american_opportunity_credit policyengine_us/tests/policy/baseline/gov/irs/credits/education/american_opportunity_credit
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/irs/credits/education -c policyengine_us

Follow-up data PR: PolicyEngine/policyengine-us-data#843

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.47%. Comparing base (8675371) to head (ddf1767).
⚠️ Report is 103 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #8170      +/-   ##
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- Coverage   100.00%   94.47%   -5.53%     
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  Files            3       18      +15     
  Lines           39      199     +160     
  Branches         0        2       +2     
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+ Hits            39      188     +149     
- Misses           0       10      +10     
- Partials         0        1       +1     

Flag	Coverage Δ
unittests	94.47% <100.00%> (-5.53%)	⬇️

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