Normalize `created` timestamp to seconds in response from proxy handlers

[iamjimyes]

This commit adjusts the created field in both NonStreamingChatProxyHandler and StreamingChatProxyHandler to use a timestamp in seconds rather than milliseconds. It applies Math.floor(Date.now()/1000) to ensure the created timestamp is consistent with OpenAI's response, enhancing compatibility, especially to response handlers in strongly typed language like rust

[zuisong]

thanks