[ML] Make assertBusy() really wait in full cluster restart tests (#93641) (#93646)

The exception being thrown that we want to ignore for 90 seconds
is not an AssertionError, so assertBusy was not looping for it.

Fixes #93325

Author: droberts195

x-pack/qa/full-cluster-restart/src/javaRestTest/java/org/elasticsearch/xpack/restart/MLModelDeploymentFullClusterRestartIT.java

@@ -13,6 +13,7 @@
 import org.elasticsearch.Version;
 import org.elasticsearch.client.Request;
 import org.elasticsearch.client.Response;
+import org.elasticsearch.client.ResponseException;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.util.concurrent.ThreadContext;
 import org.elasticsearch.common.xcontent.support.XContentMapValues;
@@ -105,8 +106,14 @@ public void testDeploymentSurvivesRestart() throws Exception {
             }));
             waitForDeploymentStarted(modelId);
             assertBusy(() -> {
-                assertInfer(modelId);
-                assertNewInfer(modelId);
+                try {
+                    assertInfer(modelId);
+                    assertNewInfer(modelId);
+                } catch (ResponseException e) {
+                    // assertBusy only loops on AssertionErrors, so we have
+                    // to convert failure status exceptions to these
+                    throw new AssertionError("Inference failed", e);
+                }
             }, 90, TimeUnit.SECONDS);
             stopDeployment(modelId);
         }