Make `Message` `createdAt` key required

[brian-xu-vlt]

Feature Description

• At the moment Message createdAt is optional

Use Case

• Making it required would avoid having to deal with possibly missing createdAt date when handling list of messages in our backend:
  • as there is no guaranty the array of messages is sorted in the order they were generated at all time, when looking for the last message from a specific role, the optional gets in the way

Additional context

Proposal PR: #2185

[lgrammel]

This would be a breaking change for existing users of Message

[lgrammel]

ai sdk 5 uses application specific, generic metadata for things such as createdAt