Fix a test error (CompletionFunction are folded before the optimizer is applied)

Author: afoucret

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/OptimizerVerificationTests.java

@@ -228,14 +228,14 @@ public void testRemoteEnrichAfterCoordinatorOnlyPlans() {
 
         err = error("""
             FROM test
-            | COMPLETION language_code = "some prompt" WITH { "inference_id" : "completion-inference-id" }
+            | COMPLETION language_code = CONCAT("some prompt: ", first_name) WITH { "inference_id" : "completion-inference-id" }
             | ENRICH _remote:languages ON language_code
             """, analyzer);
         assertThat(
             err,
             containsString(
                 "ENRICH with remote policy can't be executed after "
-                    + "[COMPLETION language_code = \"some prompt\" WITH { \"inference_id\" : \"completion-inference-id\" }]@2:3"
+                    + "[COMPLETION language_code = CONCAT(\"some prompt: \", first_name) WITH { \"inference_id\" : \"completion-inference-id\" }]@2:3"
             )
         );