Fix VT capital gains exclusion for financial instruments

[MaxGhenis]

Closes #7291

Summary

• Vermont's 40% capital gains exclusion now correctly excludes financial instruments (stocks/bonds) from eligibility
• Only the flat $5,000 exclusion applies to financial instrument gains since standard capital gains inputs represent financial instruments
• Added new input variable long_term_capital_gains_on_assets_eligible_for_vt_exclusion to allow users to model the 40% exclusion on qualifying assets (farms, businesses, timber)

Changes

• Modified vt_percentage_capital_gains_exclusion to only consider explicitly eligible capital gains
• Modified vt_capital_gains_exclusion to properly document the distinction between flat and percentage exclusions
• Added new person-level input variable for VT-eligible capital gains
• Updated tests to reflect correct behavior
• Added integration test case from the issue

Test plan

• All 149 VT tests pass
• Integration test verifies only $5,000 flat exclusion applies to financial instrument gains
• Percentage exclusion test verifies 40% only applies to explicitly eligible gains

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (66ccff2) to head (f079115).
⚠️ Report is 439 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #7300       +/-   ##
============================================
+ Coverage   72.72%   100.00%   +27.27%     
============================================
  Files        3757         3     -3754     
  Lines       54115        45    -54070     
  Branches      267         0      -267     
============================================
- Hits        39353        45    -39308     
+ Misses      14742         0    -14742     
+ Partials       20         0       -20     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+27.27%)	⬆️

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