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 (02619df) to head (3e7a7c6).
⚠️ Report is 619 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #7278   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         1    -3     
  Lines           92        17   -75     
  Branches         0         1    +1     
=========================================
- Hits            92        17   -75     

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

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