OpenAI: Use Structured Outputs

[MrSerth]

OpenAI recently introduced Structured Outputs in their API, allowing developers to specify a JSON schema that the response should adhere to. For CodeHarbor, that could be useful to split a potential code response more easily from an explanation when generating unit tests.

The ruby-openai gem we use currently discusses the possibility to integrate structured outputs natively: https://github.com/alexrudall/ruby-openai/issues/508.