Model API: ChatMessage now includes an id field

[jjallaire-aisi]

The id defaults to shortuuid.uuid() and is assigned at creation.

The id is preserved during ordinary course of business transformations including:

1. When messages are combined to meet the constraints of APIs which prohibit consecutive user or assistant messages (in which case the id of the first message is used).
2. When messages are edited to base64 encode images or de-duplicate content.