Function parse_policy_expr 

pub fn parse_policy_expr(sql: &str) -> Result<Expr, String>

Parse a raw SQL policy expression from pg_policies into a typed Expr AST.

Handles the common RLS patterns:

• col = current_setting('var', true)::type (tenant check)
• (current_setting('var', true))::boolean = true (session bool check)
• expr1 OR expr2 / expr1 AND expr2 (combinators)

Returns an error for unsupported expressions.