class based model for chat

[JGoutin]

More and more models now have specific behaviors and parameters (e.g., reasoning or caching configuration), even with the use of the standardized "Converse" Bedrock API.

Refactor to use a class-based model handling (like already done for image and embedding), but with a generic default that matches common models.

This will be clearer and more maintainable than handling exceptions in the common code.