按 priority 升序 fallback。常用配置：
· 火山豆包 base_url = https://ark.cn-beijing.volces.com/api/coding/v3，model = Doubao-Seed-2.0-pro，extra_params = {"thinking": false}
· DeepSeek base_url = https://api.deepseek.com/v1，model = deepseek-chat