feat: Support Predicted Outputs in openai_dart

[davidmigloz]

Predicted Outputs enable you to speed up API responses from Chat Completions when many of the output tokens are known ahead of time. This is most common when you are regenerating a text or code file with minor modifications. You can provide your prediction using the prediction request parameter in Chat Completions.

Predicted Outputs are available today using the latest gpt-4o and gpt-4o-mini models. Read on to learn how to use Predicted Outputs to reduce latency in your applicatons.

More info: https://platform.openai.com/docs/guides/predicted-outputs

Example:

const codeContent = '''
class User {
  firstName: string = "";
  lastName: string = "";
  username: string = "";
}

export default User;
''';

const request = CreateChatCompletionRequest(
  model: ChatCompletionModel.model(
    ChatCompletionModels.gpt4o,
  ),
  messages: [
    ChatCompletionMessage.user(
      content: ChatCompletionUserMessageContent.string(
        'Replace the username property with an email property. '
        'Respond only with code, and with no markdown formatting.',
      ),
    ),
    ChatCompletionMessage.user(
      content: ChatCompletionUserMessageContent.string(codeContent),
    ),
  ],
  prediction: PredictionContent(
    content: PredictionContentContent.text(codeContent),
  ),
);
final res1 = await client.createChatCompletion(request: request);
final choice1 = res1.choices.first;
print(choice1.message.content);
// class User {
//   firstName: string = "";
//   lastName: string = "";
//   email: string = "";
// }
// 
// export default User;

print(res1.usage?.completionTokensDetails?.acceptedPredictionTokens)
// 18
print(res1.usage?.completionTokensDetails?.rejectedPredictionTokens)
// 10