Apply suggestions from code review

Co-authored-by: Copilot <[email protected]>

Author: masseyke

rest-api-spec/src/main/resources/rest-api-spec/api/simulate.ingest.json

@@ -42,7 +42,7 @@
       "merge_type":{
         "type":"string",
         "description":"The mapping merge type if mapping overrides are being provided in mapping_addition, index_template_substitutions, or component_template_substitutions. The allowed values are one of index or template. The index option merges mappings the way they would be merged into an existing index. The template option merges mappings the way they would be merged into a template.",
-        "default": "mapping_update"
+        "default": "index"
       }
     },
     "body":{

server/src/main/java/org/elasticsearch/action/bulk/TransportSimulateBulkAction.java

@@ -176,7 +176,7 @@ private MapperService.MergeReason getMergeReason(String mergeType) {
         return Optional.ofNullable(mergeType).map(type -> switch (type) {
             case "index" -> MapperService.MergeReason.MAPPING_UPDATE;
             case "template" -> MapperService.MergeReason.INDEX_TEMPLATE;
-            default -> throw new IllegalArgumentException("Unsupported merge type " + mergeType);
+            default -> throw new IllegalArgumentException("Unsupported merge type '" + mergeType + "'. Valid values are 'index' and 'template'.");
         }).orElse(MapperService.MergeReason.MAPPING_UPDATE);
     }