Data Services: Fix @Where aggregate subquery translation generating invalid SQL #16807

Split from monolithic audit #16674 (finding F21).

Problem Description

In GORM Data Services, using an aggregate expression inside @Where:

@Where({ amount > avg(amount) })

generates invalid SQL because the aggregate projection (avg(amount)) is applied directly to the outer WHERE clause instead of being encapsulated in a subquery (WHERE amount > (SELECT avg(amount) FROM ...)).

Expected Behavior

The AST transformation for @Where should properly emit a subquery expression when an aggregate function is referenced on the right-hand side of a comparison operator.