chore(api): document stored chat completion errors (#3963)

## Summary

Clarifies error responses for stored chat completions without changing
Python exception handling.

## Changes

- Document applicable lookup, validation, rate-limit and service errors.
- Preserve existing methods, return types and exception behavior.

Co-authored-by: markstuart-oai <323302876+markstuart-oai@users.noreply.github.com>