[Feature]: 允许为某个模型单独指定 endpoint 和 api_key

### Class | 类型

程序主体

### Feature Request | 功能请求

当通过 OpenAI 兼容 API 接入多个模型时，不同模型所需的 endpoint 可能不一致。当前程序只能给 OpenAI API 配置一个全局 endpoint，无法为某个模型配置对应的 endpoint 和 api_key。

Start by locating the configuration and OpenAI-compatible API entry points that currently provide a global endpoint and api_key. Trace how model selection reaches those settings, then define completion as allowing each model to use its own endpoint and api_key while preserving existing global configuration behavior.

Written by the indexing model from the issue text.