Reenable Index Compatibility BWC Tests (elastic#134784)

This change reenables N-2 BWC tests on main that were not reenabled in elastic#125296.

Tests are running on 9.0 though.

Author: tlrx

qa/lucene-index-compatibility/build.gradle

@@ -14,6 +14,7 @@ buildParams.bwcVersions.withLatestReadOnlyIndexCompatible { bwcVersion ->
   tasks.named("javaRestTest").configure {
     systemProperty("tests.minimum.index.compatible", bwcVersion)
     usesBwcDistribution(bwcVersion)
+    enabled = true
   }
 }
 

qa/lucene-index-compatibility/src/javaRestTest/java/org/elasticsearch/lucene/AbstractIndexCompatibilityTestCase.java

@@ -26,7 +26,9 @@
 import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.test.XContentTestUtils;
 import org.elasticsearch.test.cluster.ElasticsearchCluster;
+import org.elasticsearch.test.cluster.local.DefaultSettingsProvider;
 import org.elasticsearch.test.cluster.local.LocalClusterConfigProvider;
+import org.elasticsearch.test.cluster.local.LocalClusterSpec;
 import org.elasticsearch.test.cluster.local.distribution.DistributionType;
 import org.elasticsearch.test.cluster.util.Version;
 import org.elasticsearch.test.rest.ESRestTestCase;
@@ -78,6 +80,16 @@ public abstract class AbstractIndexCompatibilityTestCase extends ESRestTestCase
         .setting("xpack.security.enabled", "false")
         .setting("xpack.ml.enabled", "false")
         .setting("path.repo", () -> REPOSITORY_PATH.getRoot().getPath())
+        .settings(new DefaultSettingsProvider() {
+            @Override
+            public Map<String, String> get(LocalClusterSpec.LocalNodeSpec nodeSpec) {
+                var settings = super.get(nodeSpec);
+                if (nodeSpec.getVersion().onOrAfter(Version.fromString("9.2.0"))) {
+                    settings.put("xpack.inference.endpoint.cache.enabled", "false");
+                }
+                return settings;
+            }
+        })
         .apply(() -> clusterConfig)
         .build();
 

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/registry/ClearInferenceEndpointCacheAction.java

@@ -104,6 +104,15 @@ public ClearInferenceEndpointCacheAction(
         );
     }
 
+    @Override
+    protected void doExecute(Task task, Request request, ActionListener<AcknowledgedResponse> listener) {
+        if (inferenceEndpointRegistry.cacheEnabled() == false) {
+            ActionListener.completeWith(listener, () -> AcknowledgedResponse.TRUE);
+            return;
+        }
+        super.doExecute(task, request, listener);
+    }
+
     @Override
     protected void masterOperation(
         Task task,