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-cli/src/lib.rs

@@ -831,6 +831,7 @@ fn warning_kind(kind: WarningKind) -> &'static str {
         WarningKind::MalformedOrUnrelatedPacket => "malformed_or_unrelated_packet",
         WarningKind::WrongToken => "wrong_token",
         WarningKind::UntrackedReply => "untracked_reply",
+        _ => "unknown",
     }
 }