fix(api): prioritize Python request routing fields (#4014)

## Summary

Prioritizes `model`, `stream`, and `service_tier` in Python JSON request
bodies, placing them before potentially large `input` or `messages`
fields.

## Changes

- Applies only to methods with one unambiguous `application/json`
request body.
- Preserves the relative order and values of all other request-body
fields.
- Leaves public method signatures and omitted-field behavior unchanged.
- Leaves multipart, form-encoded, and mixed-media requests unchanged.
- Shortens the `SessionTurnError` error-category description.

Co-authored-by: jbeckwith-oai <269036200+jbeckwith-oai@users.noreply.github.com>