Set manageDocumentIds default to true for backward compatibility

The manageDocumentIds flag was initially set to false, which would
break existing users who rely on explicit document ID management.
This change sets the default to true to preserve the current behavior
for all existing OpenSearch users.

AWS OpenSearch Serverless users can explicitly opt-in by setting
manageDocumentIds(false) when they need auto-generated IDs due to
the platform's restrictions on custom document IDs.

This ensures backward compatibility while still providing the
flexibility needed for AWS Serverless environments.

Related: gh-3818
Signed-off-by: sanghun <vitash1215@gmail.com>

Author: lsh1215

vector-stores/spring-ai-opensearch-store/src/main/java/org/springframework/ai/vectorstore/opensearch/OpenSearchVectorStore.java

@@ -147,6 +147,7 @@
  * @author Christian Tzolov
  * @author Thomas Vitale
  * @author inpink
+ * @author Sanghun Lee
  * @since 1.0.0
  */
 public class OpenSearchVectorStore extends AbstractObservationVectorStore implements InitializingBean {