Add general partner SE income to self-employment tax base

[MaxGhenis]

Problem

The self-employment tax base per 26 USC 1402(a) should include general partners' distributive share of partnership trade/business income. Currently we exclude all partnership income from SE tax.

What the law says

SE income includes:

• ✅ Schedule C net profit (self_employment_income) - implemented
• ✅ Schedule F net profit (farm_income) - implemented
• ❌ General partners' distributive share of partnership ordinary income - not implemented
• ✅ S-corp distributions excluded - fixed in PR Remove s_corp_self_employment_income from SE tax base #7237
• ✅ Limited partners excluded (except guaranteed payments) - correct by omission

Current behavior

taxable_self_employment_income only includes:

SEI_SOURCES = [
    "self_employment_income",
    "farm_income",
]

This understates SE tax for general partners who have partnership trade/business income.

Implementation

Depends on PolicyEngine/policyengine-us-data#480 to provide data distinguishing:

• General vs limited partner status
• Active trade/business vs passive investment partnership income

Once data is available, add to taxable_self_employment_income:

SEI_SOURCES = [
    "self_employment_income",
    "farm_income",
    "general_partner_se_income",  # new variable
]

Related issues

• Remove s_corp_self_employment_income variable and update SE tax base #5273 - Remove S-corp from SE tax (PR Remove s_corp_self_employment_income from SE tax base #7237) ✅
• Rename partnership_s_corp_income to partnership_s_corp_distributions #5272 - Rename partnership_s_corp_income
• Add passive activity loss limitation rules #5274 - Passive activity loss rules (Section 469) - separate issue for income tax
• Add general partner vs limited partner distinction for SE tax policyengine-us-data#480 - Data needed for partner type distinction