Preserve refusals in OpenAI Chat Generators #13255

Currently, both OpenAIChatGenerator and OpenAIResponsesChatGenerator do not preserve refusals from the API.

We should preserve this information in ChatMessage.

At the moment, this field is only handled in to_openai_dict_format/from_openai_dict_format (the latter is not used in Chat Generators). When we do the above work, we might also decide to simplify/remove support for this field from these conversion functions.

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