Rename `partnership_s_corp_income` to `partnership_s_corp_distributions`

[MaxGhenis]

The variable name partnership_s_corp_income could incorrectly suggest inclusion of S-corp W-2 wages. Reviewing the PUF documentation (field E26270), this variable only includes distributions from partnerships and S-corporations, not W-2 wages.

Proposed changes:

• Rename variable to partnership_s_corp_distributions
• Update description to clarify it contains only distributions
• Update any references in formulas/tests