fix: improve SSE event handling to gracefully ignore unrecognized events (#423)

- Replace McpError exceptions with debug/warning logs for unrecognized SSE event types
- Continue processing instead of failing when encountering unknown SSE events
- Update transport implementations:
  - WebClientStreamableHttpTransport: return empty tuple instead of throwing
  - WebFluxSseClientTransport: complete stream instead of erroring
  - HttpClientSseClientTransport: call sink.success() instead of sink.error()
  - HttpClientStreamableHttpTransport: return empty Flux for unknown events

This improves client resilience when servers send non-standard or future SSE event types.

Resolves #272 , #223 , #93, #421

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

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

@@ -366,10 +366,8 @@ public class HttpClientSseClientTransport implements McpClientTransport {
 								return Flux.just(message);
 							}
 							else {
-								logger.error("Received unrecognized SSE event type: {}",
-										responseEvent.sseEvent().event());
-								sink.error(new McpError(
-										"Received unrecognized SSE event type: " + responseEvent.sseEvent().event()));
+								logger.debug("Received unrecognized SSE event type: {}", responseEvent.sseEvent());
+								sink.success();
 							}
 						}
 						catch (IOException e) {

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

@@ -264,6 +264,10 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
 										"Error parsing JSON-RPC message: " + responseEvent.sseEvent().data()));
 							}
 						}
+						else {
+							logger.debug("Received SSE event with type: {}", responseEvent.sseEvent());
+							return Flux.empty();
+						}
 					}
 					else if (statusCode == METHOD_NOT_ALLOWED) { // NotAllowed
 						logger.debug("The server does not support SSE streams, using request-response mode.");