fix: eliminate MT federal income tax deduction for 2024+

[MaxGhenis]

Summary

• Eliminate Montana federal income tax deduction for 2024+ per Senate Bill 399
• Montana simplified its tax system in 2024 to use federal taxable income as the starting point, eliminating the separate itemized deduction for federal income taxes paid
• Add applies.yaml parameter that switches to false for 2024-01-01
• Update mt_federal_income_tax_deduction_unit.py to check this parameter and return 0 when it doesn't apply

Fixes #6973

Test plan

• Added test case for 2024 joint filers verifying deduction is $0
• Verified all existing 2021 tests still pass
• Ran make format

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (66ccff2) to head (3e7a7c6).
⚠️ Report is 430 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #7278       +/-   ##
============================================
+ Coverage   72.72%   100.00%   +27.27%     
============================================
  Files        3757         1     -3756     
  Lines       54115        17    -54098     
  Branches      267         1      -266     
============================================
- Hits        39353        17    -39336     
+ Misses      14742         0    -14742     
+ Partials       20         0       -20     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+27.27%)	⬆️

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