Remove unnecessary json decode-encode

[odp]

The outer chat_completion code already decodes result["response_body"]

    @app.post("/sessions/{session_id}/v1/chat/completions")
    async def chat_completions(request: Request, session_id: str):

            response = orjson.loads(result["response_body"])
            ...
            return backend.build_proxy_response(result)

then we do decode and re-encode the bytes again inside build_proxy_response. This is in the hot-path.

also move, json parsing to_thread, can block the event loop.

[flukeskywalker]

Looks like a clear win