Fix RI reform exports to match standard pattern

[DTrim99]

Summary

• Fixes RI high earner tax and social security exemption reform exports to match standard pattern used by RI CTC
• Changes __init__.py files to only export factory function, not pre-built reform

Test plan

• RI high earner tax tests pass
• RI social security exemption tests pass

Closes #7327

🤖 Generated with Claude Code

[DTrim99]

The issue was that the __init__.py files were exporting both the factory function and pre-built reform, which differs from the working pattern used by RI CTC.

Before (broken):

from policyengine_us.reforms.states.ri.high_earner_tax.ri_high_earner_tax_reform import (
    create_ri_high_earner_tax_reform,
    ri_high_earner_tax,
)

After (fixed):

from .ri_high_earner_tax_reform import create_ri_high_earner_tax_reform

This matches the pattern in policyengine_us/reforms/states/ri/ctc/__init__.py which works correctly.