Encapsulate more model logic in ChatMessage

What does this MR do and why?

Refactors ChatMessage logic. Key changes:

• Role validation moved to the model
• Errors are not squashed into single string anymore.
• Introduced Ai message factory.
• ChatMessage is aware of message owner by method user
• message.save! method.

How to set up and validate locally

1. Use any AI feature.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.