Preserve emoji characters in structured output payloads

[jonwiggins]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

• stop relying on httpx’s JSON serializer so we can send request bodies with ensure_ascii=False
• encode structured-output payloads as UTF-8 bytes while keeping the Content-Type: application/json header
• add a regression test that captures the chat.completions.parse request and verifies the schema still contains plain emojis

Additional context & links

Structured outputs that include Pydantic schemas were being serialized with ensure_ascii=True, so emojis (and other non-BMP characters) arrived as surrogate escapes (e.g., \ud83d\ude0d). After this change the payload stays UTF‑8, preserving the characters end-to-end.