【model-router】AI 代理接口透传不支持非 json 类型 body 的请求

[daixijun]

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如 openai 的 Speech to text 功能， Create transcription 和 Create translation 接口需要上传媒体文件，所以他的 body 格式为 form-data, 无法正确解析到 model 导致路由匹配失败。

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