[Bug]: 接入凭证就有问题，为什么只限定地址和格式

### Prerequisites

- [x] I have searched the existing [issues](https://github.com/agentscope-ai/agentscope/issues) and [discussions](https://github.com/agentscope-ai/agentscope/discussions), and this is not a duplicate.
- [x] This is a bug, not a usage question. (For questions, please use [Discussions](https://github.com/agentscope-ai/agentscope/discussions/new?category=general) instead.)

### Background / Description

你们现在这凭证添加方式非常不合理，对应的供商规定了格式。像deepseek的最新flash是https://api.deepseek.com/anthropic这种方式，你们就无法支持，添加了只能是openai格式，导致无法使用。目前我通过ai修改了底层model可以使用了。希望后续放开请求方式，而不是固定一个供应商的请求方式。这样你们也不好维护。交给前台添加的人就行了。这样供应商怎么改与你后台无关。

### Error Messages

```shell
统一个供应商无法选取不同的请求方式比如deepseek可以是：anthropic也可以是Openai格式，最新的flash只能是anthropic
```

### Steps to Reproduce

修改_base.py,_model.py添加

### Environment

- AgentScope Version:最新仓库版本
- Python Version:3.14
- OS:win

Check the provider request flow in the referenced `_base.py` and `_model.py`, focusing on where request format is fixed to OpenAI style. The first step is to read those files and trace how a model client is built for deepseek, then identify the format-selection logic. No tests are named in the issue; validation is manual by reproducing with a deepseek flash endpoint. Done means the same endpoint works with an `anthropic` request mode without hard-coding a single vendor format.

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