Backdate Montana TANF parameters

[hua7450]

Summary

Backdates Montana TANF parameters to 2016+, fixes FPL year pinning bug, and improves code quality.
Closes #7502

Changes

• Parameters backdated: payment_fpg_rate.yaml -- added 2017-07-01 entry (0.35)
• New parameters: benefit_fpg_year.yaml and payment_fpg_year.yaml -- pins benefit/payment standards to specific FPL year instead of using current-year FPL
• Formula fixes: mt_tanf_benefit_standard.py and mt_tanf_payment_standard.py rewritten to use pinned FPL year from new parameters. Fixes incorrect dollar amounts when current-year FPL differs from pinned year (~$24-28/mo overstatement for family of 3)
• Bug fix: Benefit standard eligibility test changed from <= to strict < per State Plan Developer meeting agenda 2021-08-14 #25 p.10
• Reference fixes: ARM 37.78.402 for unearned income (was 37.78.416), added ARM 37.78.420 citations
• Tests: 40 new test cases (10 historical/boundary + 30 edge cases), 158 total passing

Regulatory Sources

• MT TANF State Plan #25 (2024-2026)
• TANF 001 Monthly Income Standards
• ARM 37.78.420 -- Assistance Standards
• ARM 37.78.402 -- Treatment of Income
• 2016 TANF Brochure (Legislative Fiscal Division)

Review Summary

• Review-fix loop: 1 round, 0 critical issues remaining
• 80 dollar amounts verified against TANF 001 tables (4 standards x 20 family sizes)
• 10 core parameters confirmed correct against State Plan and ARM
• Validation checkpoint: PASS (no hardcoded values, no year-check conditionals, clean parameter-driven formulas)

Should Address (Future PRs)

• Interest income blanket inclusion (ARM says interest on countable resources should be excluded)
• Child support disregard for obligated amounts
• 60-month time limit implementation
• Pre-2015 FPL data backfill (needed for 2011/2002 FPL year pinning to work historically)

Test Plan

• All 158 existing tests pass
• 10 new historical/boundary tests pass
• 30 new edge case tests pass
• Microsimulation check (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (55a0045) to head (d841c17).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #7503      +/-   ##
===========================================
+ Coverage   97.82%   100.00%   +2.17%     
===========================================
  Files           3         4       +1     
  Lines          92        71      -21     
  Branches        2         0       -2     
===========================================
- Hits           90        71      -19     
+ Misses          1         0       -1     
+ Partials        1         0       -1     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+2.17%)	⬆️

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