Update the Mississippi HSA test for joint-column combination (main is red)

[MaxGhenis]

Summary

Main has failed one Mississippi test since #9865 merged:

ms_health_savings_account_adjustment.yaml
  'Without per-person amounts the head takes the deduction, even above the head's own income':
  ms_agi@2025: [-2000. 50000.] differs from [0. 50000.]

#9865 (subtract the HSA deduction once, from the account holder) and #9885 (combine a negative Mississippi joint column with the other column) each passed CI on their own. #9885 landed first and removed the zero floor on the head's and spouse's ms_agi. A negative column now stays negative, and ms_income_tax_before_credits_joint combines it with the other column, per Form 80-100 line 17. #9865's test still expected the old floor ("2,000 of the deduction goes unused").

The bisect is on current main:

• the test passes at Subtract Mississippi's HSA deduction once, from the account holder #9865's head d5e8301b9c;
• it fails at its merge commit b7aaf015cf, and at every later main commit.

This PR changes the test only, not the model:

• ms_agi becomes [-2_000, 50_000].
• The comment now explains the column combination.
• A new assertion pins the result of the combination. ms_income_tax_before_credits_unit is 941.60: combined Column A taxable income is 1,000 + 33,400 − 3,000 = 31,400, and (31,400 − 10,000) × 4.4% = 941.60. Without the HSA, the tax is 1,029.60. All $3,000 of the deduction reduces combined taxable income.

Tests

ms_health_savings_account_adjustment.yaml: 9 passed. Before this change, 1 failed and 8 passed on main.

Impact

No model output change: only a test file and a changelog fragment change.

axiom: n/a

🤖 Generated with Claude Code

[MaxGhenis]

US + core hub merge audit, us#9992 at 03e40e08b7 (squash). This fixes a red main.

• Cause: Subtract Mississippi's HSA deduction once, from the account holder #9865 and Combine a negative Mississippi joint column with the other column #9885 each passed CI on its own; together they broke one Mississippi test. The hub bisected:
  • the test passes at Subtract Mississippi's HSA deduction once, from the account holder #9865's head d5e8301b9c;
  • it fails at Subtract Mississippi's HSA deduction once, from the account holder #9865's merge commit b7aaf015cf, and at every later main commit.
• Change: the test only. ms_agi becomes [-2,000, 50,000]. A new assertion checks ms_income_tax_before_credits_unit = 941.60.
• Independent review: an in-session Opus 5.5 agent returned APPROVE at 03e40e0.
  • It traced the model: the floor removed in Combine a negative Mississippi joint column with the other column #9885, the column combination in the joint tax, and that separate filing (1,394.80) isn't chosen.
  • It checked 941.60 against the 2025 rate, joint exemption and standard deduction parameters, and against the Form 80-100 line 17 instructions.
  • It ran the file: 9/9 at the head, versus 1 failure on main. The whole Mississippi tax folder passes 158/158.
• Gates checked live at merge:
  • gh pr checks exits 0;
  • MERGEABLE;
  • not a draft;
  • no CHANGES_REQUESTED review;
  • head pinned.
• Impact: none. Only a test file and a changelog fragment change.
• Follow-up: two stale comments still mention per-person MS AGI floors: tests/core/test_ms_health_savings_account_adjustment_invariants.py:11 and test_dependent_above_the_line_deductions_invariants.py:546. Their assertions pass.
• --admin: used only because a required approving review is the sole block.