Function parse_bool_expr 

pub fn parse_bool_expr(
    tokens: &[Token],
    fallback_span: Span,
) -> Result<BoolExpr>

Parse a slice of schema tokens into a validated BoolExpr tree.

The token slice should contain only the expression tokens (i.e. without the surrounding @check( … ) scaffolding).

fallback_span is used for error reporting when the slice is empty.