[Bug] The environment variable DEFAULT_MODEL configuration is invalid

### 📦 Deployment Method

Vercel

### 📌 Version

v2.15.8

### 💻 Operating System

Other

### 📌 System Version

vercel

### 🌐 Browser

Chrome

### 📌 Browser Version

131.0.6778.140

### 🐛 Bug Description

```
fetch("/api/config", {
method: "post",
body: null,
headers: {
...getHeaders(),
},
})
.then((res) => res.json())
.then((res) => {
const defaultModel = res.defaultModel ?? "";
if (defaultModel !== "") {
const [model, providerName] = getModelProvider(defaultModel);
DEFAULT_CONFIG.modelConfig.model = model;
DEFAULT_CONFIG.modelConfig.providerName = providerName as any;
}

return res;
})
```

配置了环境变量DEFAULT_MODEL=gpt-3.5-turbo@OpenAi ，发现不生效。
看来源码，是有处理的。

最后定位到这里，发现是接口返回数据并没有写入。所以取数据还是默认值gpt-4o-mini

### 📷 Recurrence Steps

_No response_

### 🚦 Expected Behavior

_No response_

### 📝 Additional Information

_No response_