feat(client): add timestamp to ClientEvent::Warning

Include a timestamp when reporting warnings to allow consumers like the machine-readable formatter to provide temporal context for troubleshooting.

crates/irtt-client/src/managed/hub.rs

@@ -279,6 +279,7 @@ mod tests {
         ClientEvent::Warning {
             kind: WarningKind::UntrackedReply,
             message: format!("event-{n}"),
+            at: ClientTimestamp::now(),
         }
     }