feat: handle SSE comment messages

- Add support for ignoring SSE comment lines that start with ':'
- Add debug logging for comment line processing

Resolves #443

Signed-off-by: Christian Tzolov <[email protected]>

Author: tzolov

mcp/src/main/java/io/modelcontextprotocol/client/transport/ResponseSubscribers.java

@@ -12,6 +12,8 @@
 
 import org.reactivestreams.FlowAdapters;
 import org.reactivestreams.Subscription;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import io.modelcontextprotocol.spec.McpError;
 import reactor.core.publisher.BaseSubscriber;
@@ -31,6 +33,8 @@
  */
 class ResponseSubscribers {
 
+	private static final Logger logger = LoggerFactory.getLogger(ResponseSubscribers.class);
+
 	record SseEvent(String id, String event, String data) {
 	}
 
@@ -167,6 +171,11 @@ else if (line.startsWith("event:")) {
 						this.currentEventType.set(matcher.group(1).trim());
 					}
 				}
+				else if (line.startsWith(":")) {
+					// Ignore comment lines starting with ":"
+					// This is a no-op, just to skip comments
+					logger.debug("Ignoring comment line: {}", line);
+				}
 				else {
 					// If the response is not successful, emit an error
 					// TODO: This should be a McpTransportError