#460 error with m.get("role") when the message is a json

Description

little problem with the management of json messages. Here the fix:

`def openai_to_messages(req: dict) -> tuple[list[dict], list[dict] | None, dict]:
    """OpenAI Chat Completions -> (template messages, template tools, template kwargs)."""
    messages = []

    raw = req.get("messages", [])
    if isinstance(raw, str):
        try:
            raw = json.loads(raw)      # client double-encoded messages as a JSON string
        except ValueError:
            raw = []
    if not isinstance(raw, list):
        raw = []                       # dict / number / None -> nothing to iterate
    for m in raw:

    # for m in req.get("messages", []):
        role = m.get("role")
        if role == "developer":
            role = "system"
        out = {"role": role, "content": _parts_of(m.get("content")) if role in ("user", "tool", "assistant") else _text_of(m.get("content"))}
        if m.get("reasoning_content"):
            out["reasoning_content"] = m["reasoning_content"]
        if m.get("tool_calls"):
            calls = []
            for c in m["tool_calls"]:
                if not isinstance(c, dict):
                    continue
                fn = c.get("function", c)
                args = fn.get("arguments")
                if isinstance(args, str):               # the template requires a mapping, not a JSON string
                    args = json.loads(args) if args.strip() else {}
                calls.append({"function": {"name": fn.get("name"), "arguments": args or {}}})
            out["tool_calls"] = calls
        messages.append(out)
    tools = [t.get("function", t) if isinstance(t, dict) and t.get("type") == "function" else t
             for t in req.get("tools") or []] or None
    kwargs = {}
    # OpenAI Chat Completions: "reasoning_effort"; Responses style: "reasoning": {"effort": ...}
    reasoning = req.get("reasoning") if isinstance(req.get("reasoning"), dict) else {}
    kwargs.update(effort_kwargs(req.get("reasoning_effort") or reasoning.get("effort")))
    # the vLLM / llama.cpp convention: {"chat_template_kwargs": {"enable_thinking": false, "reasoning_effort": "low"}}
    for k, v in (req.get("chat_template_kwargs") or {}).items():
        if k == "enable_thinking" and not v:
            kwargs = {"enable_thinking": False}
        elif k == "reasoning_effort" and "enable_thinking" not in kwargs:
            kwargs.update(effort_kwargs(v))
    return _late_system_to_user(messages), tools, kwargs
`