Apply MT 2021 income tax rebate once per return, not per spouse (#9012)

[PavelMakarchuk]

Fixes #9012.

Problem

mt_non_refundable_credits is a Person-entity variable with adds = gov.states.mt.tax.income.credits.non_refundable; the 2021 list includes mt_income_tax_rebate, a TaxUnit-entity variable ($2,500 joint). The tax-unit value was projected onto every member, so a joint couple received 2 × $2,500 = $5,000 of credit — wiping liabilities up to $5,000 instead of $2,500, and even flipping the mt_files_separately election (the doubled credit zeroed the joint path first).

Per MCA 15-30-2191(1), the rebate is the lesser of the taxpayer's 2021 line-20 liability or, for a joint return, $2,500 — applied once per return.

Fix

Made mt_income_tax_rebate a Person-entity variable that splits the per-return cap across each spouse's column: joint filers get $1,250 per spouse (using the existing SEPARATE rebate amount), so the pooled person-level non-refundable credits sum to the $2,500 cap once. Single/HoH/surviving-spouse filers are unchanged. This also gives the separate-on-same-form path the correct $1,250 per-column rebate.

Tests

• Unit: mt_income_tax_rebate.yaml now verifies the joint per-return cap splits to [1_250, 1_250] (sums to $2,500, not $5,000).
• Integration: added a 2021 joint couple where the rebate reduces mt_income_tax by exactly $2,500 (not $5,000). Moved the three 2021 taxsim tests' mt_income_tax_rebate: 0 overrides from the tax-unit to the person level to match the variable's new entity.
• Full MT suite (535 tests) passes.

Independent of #9011 (MT taxable Social Security); filed separately so the two can land independently.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ff39625) to head (7353668).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

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- Hits            18        16    -2     

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unittests	100.00% <100.00%> (ø)

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