Function quote_identifiers 

pub fn quote_identifiers(
    expression: Expression,
    dialect: Option<DialectType>,
) -> Expression

Quote identifiers that need quoting based on dialect rules.

Walks the entire AST recursively and sets quoted = true on any Identifier that:

• contains special characters (anything not [a-zA-Z0-9_])
• starts with a digit
• is a SQL reserved word for the given dialect

The function takes ownership of the expression, mutates a clone, and returns the modified version.