[BUG] Minimal IndexRequest fails to serialize to JSON

What is the bug?

IndexRequest.toJsonString() fails with JsonException.

How can one reproduce the bug?

Consider this minimal JUnit 5 test:

package opensearchtest;

import static org.junit.jupiter.api.Assertions.assertNotNull;
import org.junit.jupiter.api.Test;
import org.opensearch.client.opensearch.core.IndexRequest;

public class OpenSearchSerializationTest {
    @Test
    void indexRequestToJson() {
        IndexRequest<Object> indexRequest = IndexRequest.of(i -> i
                .index("index")
                .document("document"));
        assertNotNull(indexRequest.toJsonString());
    }
}

It fails with:

jakarta.json.JsonException: Cannot find a serializer for type java.lang.String. Consider using a full-featured JsonpMapper.
	at org.opensearch.client.json.JsonpUtils$1.serialize(JsonpUtils.java:84)
	at org.opensearch.client.json.JsonpUtils.serialize(JsonpUtils.java:158)
	at org.opensearch.client.opensearch.core.IndexRequest.serialize(IndexRequest.java:288)
	at org.opensearch.client.json.PlainJsonSerializable.toJsonString(PlainJsonSerializable.java:29)
	at opensearchtest.OpenSearchSerializationTest.indexRequestToJson(OpenSearchSerializationTest.java:16)

What is your host/environment?

Java 21, opensearch-java 2.22.0

Do you have any additional context?

This is an isolated test, but in the context of our application we use toJsonString() to log the content of various requests and responses. I can probably work around with a custom serialization utility, but I was expecting this to "just work". Why expose it otherwise?

Start with the minimal JUnit 5 reproduction in the issue and trace the reported path through client/json/JsonpUtils.java, opensearch/core/IndexRequest.java, and client/json/PlainJsonSerializable.java. Confirm the behavior with Java 21 and opensearch-java 2.22.0; done means the shown IndexRequest.toJsonString() call returns JSON without JsonException.