fix: flush NATS buffer after sending completed response

The CLI was reporting deployment failures even when the listener showed
success. This was a race condition: after a successful switch deployment,
the listener would send the "completed" response then immediately signal
restart. The NATS connection closed before the buffered message was
actually sent to the broker, so the CLI never received it.

Adding Flush() after sending the completed response ensures the message
reaches NATS before the listener can exit.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/listener/listener.go

@@ -260,6 +260,10 @@ func (l *Listener) handleDeployRequest(subject string, data []byte) {
 			messages.StatusCompleted,
 			"deployment completed successfully",
 		))
+		// Flush to ensure the completed response is sent before we potentially restart
+		if err := l.client.Flush(); err != nil {
+			l.logger.Error("failed to flush completed response", "error", err)
+		}
 		if l.metrics != nil {
 			l.metrics.RecordDeploymentEnd(req.Action, true, duration)
 		}