Portkey Support

I would like to send a PR for [Portkey](https://portkey.ai/docs/product/ai-gateway/configs) support.

It would work like this:

```yaml
llm: &default_llm
provider: openai_portkey
model: gpt-5-mini
proxy_url:
# rest is similar to any regular OpenAI model
```

And basically depend on these API keys:

```bash
export PORTKEY_API_KEY=""
export PORTKEY_OPENAI_VIRTUAL_KEY=""
```

Start with the YAML configuration example and the linked Portkey API Gateway documentation, then trace how regular OpenAI models and environment-based API keys are handled. Done means the `openai_portkey` provider accepts the shown model, proxy URL, and two PORTKEY credentials and can send requests through Portkey.

Written by the indexing model from the issue text.