[Bug] Hope to be able to specify the vendor for custom models

### 📦 部署方式

Vercel

### 📌 软件版本

Latest

### 💻 系统环境

Windows

### 📌 系统版本

Any

### 🌐 浏览器

Chrome

### 📌 浏览器版本

Any

### 🐛 问题描述

Currently, all custom models are treated as "OpenAI," causing requests to be sent to the wrong place, and the API directly returns that the model cannot be found. I hope it will be possible to specify the model provider.
For example, a format like this could be used: `model@provider` or `provider:model`

### 📷 复现步骤

Open the settings, enter a custom model name, such as xAI's grok3-mini, then try chatting to see how frustrating it can be

### 🚦 期望结果

You can use a format similar to `grok3-mini@XAI` or `XAI:grok3-mini` to add custom models

### 📝 补充信息

_No response_