fix(ai): pin Treatment to tool output transport

[BakerSean168]

Summary

• pin Treatment structured output to PydanticAI ToolOutput(final_result)
• prevent OpenAI-compatible provider profiles from switching Treatment to native response_format=json_schema
• add regression coverage proving final_result tool transport is preserved even when the model profile prefers native structured output

Why

Canonical staging @ c78c7a0 still produced AI 500 during live Treatment proposal generation after bodysense-structured was moved to Gemini. Diagnosis already solved the same compatibility class by pinning ToolOutput; Treatment had not.

Verification

• treatment focused pytest: 13 passed
• Ruff: pass
• Pyright: 0 errors / 0 warnings
• git diff --check: pass

Delivery boundary

BS-UPG-110 staging requalification only. Do not deploy production.