Extend AMT kiddie tax to include full-time students under age 24

[MaxGhenis]

Summary

Extends the AMT kiddie tax to include full-time students ages 19-23, per IRC 1(g)(2)(A).

Per 26 U.S.C. § 1(g)(2)(A), the kiddie tax applies to children who meet the age requirements of Section 152(c)(3), which includes:

• Children under age 19
• Full-time students under age 24

Previously, the AMT kiddie tax only checked if the filer was under age 19.

Changes:

• Updated amt_kiddie_tax_applies to check student status for ages 19-23
• Added head_is_full_time_student and spouse_is_full_time_student variables
• Added 5 new tests for student scenarios

Closes #3909

Test plan

• Unit tests for non-student scenarios (existing, updated names)
• Unit tests for student scenarios (new)
  • 20-year-old student: kiddie tax applies
  • 23-year-old student: kiddie tax applies
  • 24-year-old student: kiddie tax does NOT apply
  • 20-year-old non-student: kiddie tax does NOT apply
  • Student couple with 24+ spouse: kiddie tax does NOT apply

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8272414) to head (6342857).
⚠️ Report is 25 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #6863   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines           16        23    +7     
=========================================
+ Hits            16        23    +7     

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

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