HttpClientStreamableTransport#sendMessage: fix race condition on 404 response

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

@@ -590,6 +590,13 @@
 						// server does not recognise is already invalidated by the time
 						// the caller learns about it. Consumed here so that it is not
 						// handled a second time below.
+						// Detached from the session first: invalidating the session
+						// closes it, which would otherwise dispose this very exchange and
+						// complete sendMessage() through doOnCancel below.
+						Disposable ref = disposableRef.getAndSet(null);
+						if (ref != null) {
+							transportSession.removeConnection(ref);
+						}
 						handleExceptionSafely(first.getThrowable());
 						deliveredSink.error(first.getThrowable());
 						return Flux.empty();
@@ -876,12 +883,12 @@
 		 * {@link #authorizationErrorHandler(McpHttpClientTransportAuthorizationErrorHandler)}
 		 */
 		@Deprecated(forRemoval = true, since = "2.0.0")
 		public Builder authorizationErrorHandler(McpHttpClientAuthorizationErrorHandler authorizationErrorHandler) {
 			this.authorizationErrorHandler = new McpHttpClientTransportAuthorizationErrorHandler() {
 				@Override
 				public Publisher<Boolean> handle(HttpRequestSnapshot requestSnapshot,
 						HttpResponse.ResponseInfo responseInfo, McpTransportContext context) {
 					return authorizationErrorHandler.handle(responseInfo, context);
 				}
 
 				@Override