Resolve types of generated structs based on their values because their types are UNDEFINED

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

opensearch/src/main/java/org/opensearch/sql/opensearch/data/value/OpenSearchExprValueFactory.java

@@ -190,7 +190,12 @@ private ExprValue parse(
 
     // Field type may be not defined in mapping if users have disabled dynamic mapping.
     // Then try to parse content directly based on the value itself
-    if (fieldType.isEmpty()) {
+    // Besides, sub-fields of generated objects are also of type UNDEFINED. We parse the content
+    // directly on the value itself for this case as well.
+    // TODO: Remove the second condition once https://github.com/opensearch-project/sql/issues/3751
+    //  is resolved
+    if (fieldType.isEmpty()
+        || fieldType.get().equals(OpenSearchDataType.of(ExprCoreType.UNDEFINED))) {
       return parseContent(content);
     }