Add Montana unemployment compensation subtraction for pre-2024 years

[DTrim99]

Fixes #9211.

Montana exempted unemployment compensation from state income tax until SB 399 (Ch. 503, L. 2021) repealed the exemption effective tax year 2024, but the model had no unemployment subtraction in any year — UI flowed into Montana AGI and, as a knock-on, inflated Montana-taxable Social Security (the Taxable Social Security Benefits worksheet subtracts the Subtractions Schedule subtotal from provisional income).

Found via PolicyEngine/policyengine-taxsim#1120 (2021 MT single filer, age 74, with $23,120 of unemployment compensation; filed by @feenberg with a TaxAct return).

Changes

• New mt_unemployment_compensation_subtraction (Person): adds = ["taxable_unemployment_compensation"], matching the Subtractions Schedule line 7 instruction to enter "the same amount" of unemployment benefits reported on federal Schedule 1, line 7 (2021 Form 2 instructions, p. 25).
• Added it to the pre-2024 subtractions.yaml lists (2010 and 2021 brackets). The 2024+ lists are unchanged — UI is taxable in Montana from 2024.
• No change needed in mt_taxable_social_security; it already subtracts mt_subtractions, so the exemption propagates into the worksheet automatically.

Validation against the TaxAct return (taxsim #1120)

Quantity	TaxAct	PE before	PE after
Federal AGI	73,013	73,013.31	73,013.31
Subtractions line 3 (interest exemption 65+)	800	800	800
Subtractions line 7 (unemployment)	23,120	0	23,120
Subtractions line 37 (SS adjustment)	20,332	680	20,332
Montana AGI	28,761	71,533.32	28,761.32
Montana taxable income	n/a*	61,373.32	18,601.32

* Only the Subtractions Schedule page of the TaxAct return was uploaded; the remaining difference between PE's itemized federal-tax deduction ($5,000) and the capped standard deduction ($4,830) is a separate, known convention question (taxsim #1112).

Emulator end-to-end on the exact TAXSIM input row reproduces the table above (tax before the HB 192 rebate: 686.08; the 2023 one-time rebate then nets 2021 liability to 0 under the taxsim #1068 booking convention).

All tests in the new file pass, plus the full gov/states/mt baseline suite.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0eec199) to head (5d99697).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #9212   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         1    -1     
  Lines           31        10   -21     
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- Hits            31        10   -21     

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

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[DTrim99]

Added a second validation record from PolicyEngine/policyengine-taxsim#1121 (2021 MT single, $50,823 unemployment-only income): with the subtraction in place, Montana AGI drops to the remaining $309.73 of interest, the standard deduction floors at the single minimum, and taxable income zeroes out — matching the TaxAct return's subtraction schedule (line 7 = 50,823).