SQLSmith/randgen only generate DEFAULT expressions that match a column's type exactly. Therefore, our randomized tests don't exercise logic in #81071 that allows creating a DEFAULT expression of type A for coluymn of type B if a cast from A to B is allowed in the assignment context.

When generating DEFAULT expressions, we should randomly pick a type for the expression that can be assignment-cast into the column's type.

Jira issue: CRDB-25097