Function validate_success_expr 

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

Validate a successExpr at admission (ADR-0005 §4/§15): 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 (e.g. snapshot.tags['x'] when the trial data lacks x) are tolerated as data-dependent, mirroring crate::identity::validate_identity_expr.