[ML] Add ElasticInferenceServiceCompletionServiceSettings (elastic#123155) (elastic#123172)

Adding the missing NamedWriteable to the registry.

Author: prwhelan

docs/changelog/123155.yaml

@@ -0,0 +1,5 @@
+pr: 123155
+summary: Add `ElasticInferenceServiceCompletionServiceSettings`
+area: Machine Learning
+type: bug
+issues: []

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

@@ -57,6 +57,7 @@
 import org.elasticsearch.xpack.inference.services.cohere.rerank.CohereRerankServiceSettings;
 import org.elasticsearch.xpack.inference.services.cohere.rerank.CohereRerankTaskSettings;
 import org.elasticsearch.xpack.inference.services.elastic.ElasticInferenceServiceSparseEmbeddingsServiceSettings;
+import org.elasticsearch.xpack.inference.services.elastic.completion.ElasticInferenceServiceCompletionServiceSettings;
 import org.elasticsearch.xpack.inference.services.elasticsearch.CustomElandInternalServiceSettings;
 import org.elasticsearch.xpack.inference.services.elasticsearch.CustomElandInternalTextEmbeddingServiceSettings;
 import org.elasticsearch.xpack.inference.services.elasticsearch.ElasticRerankerServiceSettings;
@@ -618,5 +619,12 @@ private static void addEisNamedWriteables(List<NamedWriteableRegistry.Entry> nam
                 ElasticInferenceServiceSparseEmbeddingsServiceSettings::new
             )
         );
+        namedWriteables.add(
+            new NamedWriteableRegistry.Entry(
+                ServiceSettings.class,
+                ElasticInferenceServiceCompletionServiceSettings.NAME,
+                ElasticInferenceServiceCompletionServiceSettings::new
+            )
+        );
     }
 }