Better-Auth uses nanoid strings for user IDs, not UUIDs. Changed all
user_id parameter/return types in the API layer from UUID to str,
removed the obsolete UUID import where unused, and updated the
_validate_session_token return type accordingly.
Co-authored-by: CartSnitch Engineer Bot <cartnoreply@cartsnitch.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>