Support .system role for instructions in `MLXLanguageModel` 



### Current State
- **MLXLanguageModel**: Only creates `.user` role messages, never extracts or uses session instructions
- **MLXLMCommon Support**: Full support for system messages via `Chat.Message.system(content)` or explicit [`.system`](https://github.com/ml-explore/mlx-swift-lm/blob/b857882f15c8bad077644898e0d3ab44c15c8852/Libraries/MLXLMCommon/Chat.swift#L52) role