Function validate_preflight_expr 

pub fn validate_preflight_expr(expr: &str) -> ValidationResult

Validate a preflight expression at admission: it must compile, and — because CEL reports an out-of-scope variable only at evaluation time — it must trial-evaluate against a representative environment without referencing an undeclared variable, returning a bool. Missing map keys on a data-dependent index are tolerated, mirroring crate::success_expr::validate_success_expr.