chore: add comment for closeCode guard in keepalive loop

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: SERDUN

packages/webtrit_signaling/lib/src/webtrit_signaling_client.dart

@@ -309,6 +309,7 @@ class WebtritSignalingClient {
       final elapsed = await _executeKeepaliveTransaction(defaultExecuteTransactionTimeoutDuration);
       _logger.finest('handshake keepalive latency: $elapsed');
 
+      // Stop the keepalive loop if the socket was closed between send and response.
       if (_wsc.closeCode == null) {
         _startKeepaliveTimer();
       }