Refactor settings for other providers

Hi thanks for the agent first. So far it's worked great.

I wanted to ask for a refactor of the provider settings, as they can be quite confusing, at least to me, the provider seems to represent the provider AND the model in the current implementation.

This is what I have now:
```json
{
"provider": "ds-v4-flash-0731",
"providers": {
"ds-v4-flash-0731": {
"provider_type": "openai",
"base_url": "https://api.eurouter.ai/api/v1",
"api_key": "xxx",
"model": "deepseek-v4-flash-0731"
},
"glm-5.2": {
"provider_type": "openai",
"base_url": "https://api.eurouter.ai/api/v1",
"api_key": "xxx",
"model": "glm-5.2"
},
"glm-5.3": {
"provider_type": "openai",
"base_url": "https://api.eurouter.ai/api/v1",
"api_key": "xx",
"model": "glm-5.3"
}
}
}
```

But every time I need a new model, I have to declare the whole provider, duplicating the api key and the base_url.

If you consider my suggestion, you could use something akin to `zed` instead:

```json
{
"model": "glm-5.2",
"provider": "eurouter",
"providers": {
"eurouter": {
"provider_type": "openai",
"api_url": "https://api.eurouter.ai/api/v1",
"api_key": "xxxx"
"available_models": [
{
"name": "glm-5.2",
"display_name": "GLM-5.2",
"reasoning_effort": "max",
"capabilities": {
"tools": true
}
},
{
"name": "deepseek-v4-flash-0731",
"display_name": "ds-v4-flash-0731",
"capabilities": {
"tools": true
}
}
]
}
}
}
```

Thanks again