feat: `ResponsesModel` trait

- [x] I have looked for existing issues (including closed) about this

## Feature Request

We should probably have a `ResponsesModel` trait to accommodate for the fact that more providers are now trying to use the OpenAI Responses API.

### Motivation

More provider compatibility -> more user adoption for Rig -> everyone wins

### Proposal

TBD

The issue mentions adding a `ResponsesModel` trait for OpenAI Responses API compatibility. Look at existing provider traits and models in the codebase to understand the pattern. Determine what methods the trait should define and how it integrates with the current architecture. Check for any existing work on OpenAI Responses API support.

索引モデルが issue の本文から書いたものです。