[BUG] Serverless mode: writes fail with 404 on refresh endpoint

[lawofcycles]

What is the bug?

When writing data to OpenSearch Serverless with opensearch.serverless=true, the write operation fails with a 404 error because BulkProcessor.close() calls RestClient.refreshIndex(String), which does not have the serverless mode skip logic.

`RestClient.refresh(Resource)` was correctly updated in #586 to skip the refresh call in serverless mode, but `RestClient.refreshIndex(String)` (a separate method called from `BulkProcessor`) was not updated. Since `opensearch.batch.write.refresh` defaults to `true`, all Spark writes to OpenSearch Serverless fail unless the user explicitly sets `opensearch.batch.write.refresh=false`.

How can one reproduce the bug?

Using PySpark on EMR with opensearch-hadoop 1.3.0-SNAPSHOT (commit 9807b25):

from pyspark.sql import SparkSession

spark = SparkSession.builder.getOrCreate()

data = [{"id": i, "value": f"doc_{i}"} for i in range(10)]
df = spark.createDataFrame(data)
df.write \
 .format("org.opensearch.spark.sql") \
 .option("opensearch.nodes", "<serverless endpoint>") \
 .option("opensearch.port", "443") \
 .option("opensearch.nodes.wan.only", "true") \
 .option("opensearch.net.ssl", "true") \
 .option("opensearch.aws.sigv4.enabled", "true") \
 .option("opensearch.aws.sigv4.region", "<region>") \
 .option("opensearch.resource", "test-index") \
 .option("opensearch.nodes.resolve.hostname", "false") \
 .option("opensearch.serverless", "true") \
 .mode("append") \
 .save()

This fails with a 404 on _refresh. Adding .option("opensearch.batch.write.refresh", "false") works around the issue.

What is the expected behavior?

RestClient.refreshIndex(String) should skip the refresh call when opensearch.serverless=true, consistent with RestClient.refresh(Resource).

What is your host/environment?

EMR 7.12 (Spark 3.5, JDK 17), OpenSearch Serverless (ap-northeast-1)

Do you have any screenshots?

N/A

Do you have any additional context?

Introduced in #586. The fix is to add the same serverless mode check to RestClient.refreshIndex(String) that already exists in RestClient.refresh(Resource).