Function build_default_expr 

pub fn build_default_expr(
    node: &NodeEnum,
    target_type: Option<&ColumnType>,
    default_schema: Option<&Identifier>,
    location: &SourceLocation,
) -> Result<DefaultExpr, ParseError>

Convert a column-default expression node into a DefaultExpr.

Recognizes:

• Bare AConst integer/float/string/bool/null literals → DefaultExpr::Literal.
• nextval('seq') and nextval('schema.seq') → DefaultExpr::Sequence.
• Anything else → DefaultExpr::Expr via the normalizer.

target_type enables redundant-cast stripping in the Expr arm.