BE fix for 23505: Models should not allow variables (#24902)

* Add validation to card PUT and POST endpoints to invalidate saving models with variables

* Fix validation for snippets and saved question CTEs

* Move validation to pre-update and pre-insert instead of API endpoints

* Fix lint warning

* Replace hard-coded id

* Optimize DB calls in card pre-update

* Accept formatting suggestion

Co-authored-by: Ngoc Khuat <qn.khuat@gmail.com>

Co-authored-by: Ngoc Khuat <qn.khuat@gmail.com>