Function coerce_expression 

pub fn coerce_expression(
    expr: Expression,
    scope: &DType,
) -> VortexResult<Expression>

Rewrite an expression tree to insert casts where a scalar function’s coerce_args demands a different type than what the child currently produces.

The rewrite is bottom-up: children are coerced first, then each parent node checks whether its children match the coerced argument types.