refactor: mark public enums as non-exhaustive to improve API stability

Apply the #[non-exhaustive] attribute to ClientError and WarningKind to prevent breaking changes when adding new variants. Handle the catch-all case in the CLI formatter.

crates/irtt-client/src/event.rs

@@ -101,6 +101,7 @@ pub enum ClientEvent {
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[non_exhaustive]
 pub enum WarningKind {
     MalformedOrUnrelatedPacket,
     WrongToken,