fix: add additional Wait for Ray API to be responsive

This attempts to prevent the following test failure:

```
[ERROR] io.odh.test.e2e.standard.DistributedST.testDistributedWorkloadWithKueue -- Time elapsed: 402.4 s <<< ERROR!
java.io.IOException: HTTP/1.1 header parser received no bytes
```

src/main/java/io/odh/test/platform/RayClient.java

@@ -14,6 +14,7 @@
 import lombok.SneakyThrows;
 
 import java.io.IOException;
+import java.net.HttpURLConnection;
 import java.net.URI;
 import java.net.http.HttpClient;
 import java.net.http.HttpRequest;
@@ -42,6 +43,20 @@ public RayClient(HttpClient httpClient, String baseUrl, String oauthToken) {
         this.oauthToken = oauthToken;
     }
 
+    /**
+     * Check that the API server is responsive and ready to accept jobs
+     */
+    @SneakyThrows
+    public boolean isLive() {
+        HttpRequest request = buildRequest()
+                .uri(URI.create(baseUrl + "/api/jobs/"))
+                .method("HEAD", HttpRequest.BodyPublishers.noBody())
+                .build();
+        HttpResponse<Void> result = httpClient.send(request, HttpResponse.BodyHandlers.discarding());
+
+        return result.statusCode() == HttpURLConnection.HTTP_OK;
+    }
+
     /**
      * @return Job ID
      */

src/test/java/io/odh/test/e2e/standard/DistributedST.java

@@ -38,6 +38,7 @@
 import io.skodjob.annotations.SuiteDoc;
 import io.skodjob.annotations.TestDoc;
 import io.skodjob.testframe.resources.KubeResourceManager;
+import io.skodjob.testframe.wait.Wait;
 import io.x_k8s.kueue.v1beta1.ClusterQueue;
 import io.x_k8s.kueue.v1beta1.ClusterQueueBuilder;
 import io.x_k8s.kueue.v1beta1.LocalQueue;
@@ -62,6 +63,8 @@
 import java.util.concurrent.TimeUnit;
 import java.util.function.Predicate;
 
+import static io.odh.test.TestConstants.GLOBAL_POLL_INTERVAL_1_SEC;
+import static io.odh.test.TestConstants.GLOBAL_STABILITY_TIME;
 import static io.odh.test.TestConstants.GLOBAL_TIMEOUT;
 
 @SuiteDoc(
@@ -325,11 +328,15 @@ void testDistributedWorkloadWithKueue() throws Exception {
                 .sslContext(TlsUtils.getSSLContextFromSecret(signingKey))
                 .build();
 
-        Allure.step("Wait for service availability");
+        Allure.step("Wait for OpenShift service availability");
         TestUtils.waitForServiceNotUnavailable(httpClient, url);
 
+        RayClient ray = new RayClient(httpClient, url, oauthToken);
+
+        Allure.step("Wait for Ray API availability");
+        Wait.until("Ray API is available", GLOBAL_POLL_INTERVAL_1_SEC, GLOBAL_STABILITY_TIME, ray::isLive);
+
         Allure.step("Run workload through Ray API", () -> {
-            RayClient ray = new RayClient(httpClient, url, oauthToken);
             String jobId = ray.submitJob("expr 3 + 4");
             ray.waitForJob(jobId);
             String logs = ray.getJobLogs(jobId);