Phase out the Minnesota AMT exemption at 25% instead of the federal 2026 rate

[PavelMakarchuk]

Summary

mn_amt reduced the Minnesota AMT exemption at the federal phase-out rate, gov.irs.income.amt.exemption.phase_out.rate. That rate is 50% from 2026, because P.L. 119-21 added it to IRC 55(d)(4). Minn. Stat. 290.091, subd. 3(b) says the exemption "is subject to the phase out under section 55(d)(2)" and that "section 55(d)(4) of the Internal Revenue Code does not apply". IRC 55(d)(2) uses 25%, and the 2025 Schedule M1MT, line 24, says "Multiply line 23 by 25% (.25)".

This PR adds gov.states.mn.tax.income.amt.phase_out_rate (0.25 from 2021, the first year of the other Minnesota AMT parameters) and uses it in mn_amt. Results through 2025 do not change, because the federal rate was also 25%.

Issue reproduction (couple, wages split evenly), mn_amt on main and on this branch:

Wages	Year	main	branch
$300,000	2025	0	0
$300,000	2026	192.13	0
$350,000	2026	974.06	0

Tests

Added to mn_amt.yaml (arithmetic in each case):

• 2026 joint, AMT taxable income $300,000, basic tax 0: $16,202.03 (main gives $18,733.28 at 50%). Fails on main, passes here.
• 2026 joint, $300,000, basic tax $17,000: $0 (main gives $1,733.28). Fails on main, passes here.
• 2026 single, $500,000 (exemption fully phased out at either rate): $33,750. Control.
• 2025 joint, $300,000: $16,342.43. Control.

Results: mn_amt.yaml 2 failed, 7 passed on main; 9 passed here. tests/policy/baseline/gov/states/mn/: 536 passed. Parameter code-health tests: 124 passed.

Fixes #10204

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