Expression handling in `VortexSource` isn't consistent, and might cause runtime errors or other issues.

[AdamGS]

The default behavior in can_be_pushed_down_impl treats CastExpr as supported only by the overall shape of the child, but doesn't validate its actual children.

For example WHERE CAST(a % 2 AS BIGINT) = 0 will be pushed down, but we currently can't handle %.

This is a symptom of a bigger drift between the different checks, conversions and operations like expression splitting.