Update OpenAI to use Response API

[eneufeld]

Feature Description:

The OpenAI Language Model is currently using the ChatCompletion API. It is older and does not support the latest features. We should thus update to the newer Response API

https://platform.openai.com/docs/api-reference/responses

[ndoschek]

The use of the Response API should be configurable via a user setting:

• For the official OpenAI provider, this setting should default to enabled.
• For custom providers, users must explicitly enable it per model.