[BUG] opensearch_api source: _bulk endpoint returns empty HTTP 200 with no JSON body

Describe the bug
The opensearch_api source _bulk endpoint returns an empty HTTP 200 response with no body. Clients using RestHighLevelClient (Flink OpenSearch Connector, Logstash, opensearch-py) expect a JSON response matching the OpenSearch _bulk API format: {took, errors, items[]} and fail with "Unable to parse response body" / NullPointerException.

To Reproduce

1. Configure Data Prepper with opensearch_api source
2. Send a _bulk request from any standard OpenSearch client (e.g. Flink OpenSearch Connector, opensearch-py, curl)
3. Observe empty response body - no JSON returned

Expected behavior
The _bulk endpoint should return a JSON response with {took, errors, items[]} matching the OpenSearch Bulk API response format, so standard clients can parse it without error.

Screenshots
N/A

Environment (please complete the following information):

• OS: Linux
• Version: Data Prepper 2.15.1 / main branch

Additional context
Forum report: https://forum.opensearch.org/t/data-prepper-opensearch-api-return-value-for-bulk-endpoint-is-wrong/28068

The issue is in OpenSearchAPIService.processBulkRequest() which returns HttpResponse.of(HttpStatus.OK) with no body.