[ML] Add validation for updating num_threads

docs/changelog/138122.yaml

@@ -0,0 +1,6 @@
+pr: 138122
+summary: Add validation for updating `num_threads`
+area: Machine Learning
+type: bug
+issues:
+ - 137129

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/elasticsearch/ElasticsearchInternalServiceSettings.java

@@ -234,6 +234,10 @@ public ElasticsearchInternalServiceSettings updateServiceSettings(Map<String, Ob
         var validationException = new ValidationException();
         var mutableServiceSettings = new HashMap<>(serviceSettings);
 
+        if (serviceSettings.containsKey(NUM_THREADS)) {
+            validationException.addValidationError(Strings.format("[%s] cannot be updated", NUM_THREADS));
+        }
+
         var numAllocations = extractOptionalPositiveInteger(
             mutableServiceSettings,
             NUM_ALLOCATIONS,

x-pack/plugin/inference/src/test/java/org/elasticsearch/xpack/inference/services/elasticsearch/ElasticsearchInternalServiceTests.java

@@ -18,6 +18,7 @@
 import org.elasticsearch.action.support.PlainActionFuture;
 import org.elasticsearch.client.internal.Client;
 import org.elasticsearch.cluster.service.ClusterService;
+import org.elasticsearch.common.ValidationException;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.logging.DeprecationLogger;
@@ -2205,6 +2206,17 @@ public void test_providedTimeoutPropagateProperly() throws InterruptedException
         assertEquals(providedTimeout, requestCaptor.getValue().getInferenceTimeout());
     }
 
+    public void testUpdateServiceSettings_ThrowsExceptionWhenNumThreadsIsUpdated() {
+        var existingSettings = new ElasticsearchInternalServiceSettings(1, 4, "test-model", null, null);
+
+        Map<String, Object> serviceSettingsWithNumThreads = Map.of(ElasticsearchInternalServiceSettings.NUM_THREADS, 8);
+        var exception = expectThrows(
+            ValidationException.class,
+            () -> existingSettings.updateServiceSettings(serviceSettingsWithNumThreads)
+        );
+        assertThat(exception.getMessage(), containsString("[num_threads] cannot be updated"));
+    }
+
     private ElasticsearchInternalService createService(Client client) {
         var cs = mock(ClusterService.class);
         var cSettings = new ClusterSettings(Settings.EMPTY, Set.of(MachineLearningField.MAX_LAZY_ML_NODES));