[Test] Force close of HTTP response stream

In JDK23 the `HttpServer` requires that http response be explicitly
closed (even if there is no response body)

Resolves: elastic#109452

x-pack/plugin/security/qa/saml-rest-tests/src/javaRestTest/java/org/elasticsearch/xpack/security/authc/saml/SamlServiceProviderMetadataIT.java

@@ -144,6 +144,7 @@ private static void configureMetadataResource(int realmNumber) throws Certificat
             } else {
                 if (randomBoolean()) {
                     http.sendResponseHeaders(randomFrom(404, 401, 403, 500), 0);
+                    http.getResponseBody().close();
                 } else {
                     sendXmlContent("not valid xml", http);
                 }