Feature/fixed created at type

[kayhantolga]

This is a copy of PR #570. I broke that PR while trying to resolve merge conflicts, so it was easier to create a new one and cherry-pick the changes.

AI Summary:
This pull request includes changes to multiple response models in the OpenAI.SDK to improve the handling of timestamps by converting Unix timestamps to DateTimeOffset. Additionally, new properties for temperature and top_p have been added to the AssistantResponse model.

Timestamp Handling Improvements:

• OpenAI.SDK/ObjectModels/ResponseModels/BatchResponseModel/BatchResponse.cs: Changed CreatedAt from int to CreatedAtUnix of type long and added a computed CreatedAt property of type DateTimeOffset.
• OpenAI.SDK/ObjectModels/ResponseModels/ChatCompletionCreateResponse.cs: Changed CreatedAt from int to CreatedAtUnix of type long and added a computed CreatedAt property of type DateTimeOffset.
• OpenAI.SDK/ObjectModels/ResponseModels/CompletionCreateResponse.cs: Changed CreatedAt from int to CreatedAtUnix of type long and added a computed CreatedAt property of type DateTimeOffset.
• OpenAI.SDK/ObjectModels/ResponseModels/EditCreateResponse.cs: Changed CreatedAt from int to CreatedAtUnix of type long and added a computed CreatedAt property of type DateTimeOffset.
• Similar changes applied to other response models including FileUploadResponse, FineTuneResponse, FineTuningJobResponse, ImageCreateResponse, RunStepResponse, VectorStoreFileBatchObject, VectorStoreFileObject, VectorStoreObjectResponse, AssistantFileResponse, EventResponse, FileResponse, MessageResponse, RunResponse, and ThreadResponse. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

New Properties for AssistantResponse:

• OpenAI.SDK/ObjectModels/SharedModels/AssistantResponse.cs: Added Temperature and TopP properties to control sampling behavior. [1] [2]