Function fold_constants 

pub fn fold_constants(expr: &Expr) -> Expr

Folds constant sub-expressions in a DataFusion [Expr] tree.

Handles:

• Arithmetic on literal pairs (Literal op Literal)
• Boolean identity rules (true AND x → x, false OR x → x, etc.)
• Recursive descent into BinaryExpr, Not children

Returns a (possibly simplified) clone of the input expression.