Further refine logging in StompErrorHandler

In 5.3.x and forward, completely avoid logging unsent CONNECT messages
that are most likely authentication issues before the session is even
established.

Closes gh-26026

Author: rstoyanchev

spring-websocket/src/main/java/org/springframework/web/socket/messaging/StompSubProtocolHandler.java

@@ -269,13 +269,15 @@ else if (webSocketMessage instanceof BinaryMessage) {
 		}
 
 		for (Message<byte[]> message : messages) {
-			try {
-				StompHeaderAccessor headerAccessor =
-						MessageHeaderAccessor.getAccessor(message, StompHeaderAccessor.class);
-				Assert.state(headerAccessor != null, "No StompHeaderAccessor");
+			StompHeaderAccessor headerAccessor =
+					MessageHeaderAccessor.getAccessor(message, StompHeaderAccessor.class);
+			Assert.state(headerAccessor != null, "No StompHeaderAccessor");
+
+			StompCommand command = headerAccessor.getCommand();
+			boolean isConnect = StompCommand.CONNECT.equals(command) || StompCommand.STOMP.equals(command);
 
-				StompCommand command = headerAccessor.getCommand();
-				boolean isConnect = StompCommand.CONNECT.equals(command) || StompCommand.STOMP.equals(command);
+			boolean sent = false;
+			try {
 
 				headerAccessor.setSessionId(session.getId());
 				headerAccessor.setSessionAttributes(session.getAttributes());
@@ -305,7 +307,7 @@ else if (StompCommand.DISCONNECT.equals(command)) {
 
 				try {
 					SimpAttributesContextHolder.setAttributesFromMessage(message);
-					boolean sent = outputChannel.send(message);
+					sent = outputChannel.send(message);
 
 					if (sent) {
 						if (this.eventPublisher != null) {
@@ -327,13 +329,14 @@ else if (StompCommand.UNSUBSCRIBE.equals(command)) {
 				}
 			}
 			catch (Throwable ex) {
-				if (logger.isErrorEnabled()) {
-					String errorText = "Failed to send message to MessageChannel in session " + session.getId();
-					if (logger.isDebugEnabled()) {
-						logger.debug(errorText, ex);
-					}
-					else {
-						logger.error(errorText + ":" + ex.getMessage());
+				if (logger.isDebugEnabled()) {
+					logger.debug("Failed to send message to MessageChannel in session " + session.getId(), ex);
+				}
+				else if (logger.isErrorEnabled()) {
+					// Skip unsent CONNECT messages (likely auth issues)
+					if (!isConnect || sent) {
+						logger.error("Failed to send message to MessageChannel in session " + session.getId() +
+								":" + ex.getMessage());
 					}
 				}
 				handleError(session, ex, message);