[BUG] OpenSearch Serverless Sink does not create index on startup

Describe the bug
When using an OpenSearch Serverless collection in a pipeline in the opensearch sink, the index configured is not created until a document is sent to the pipeline and is indexed into the collection.

To Reproduce
Steps to reproduce the behavior:

1. Create a pipeline with an opensearch serverless collection as a sink
2. Observe the index configured in the sink is not created when starting Data Prepper
3. Send an Event to the pipeline
4. Observe the document in the collection in the specified index

Expected behavior
The index should be created on startup like it is for non-serverless opensearch sinks

Screenshots
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Environment (please complete the following information):

• OS: [e.g. Ubuntu 20.04 LTS]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.

Start at the OpenSearch sink startup path and reproduce the issue with an OpenSearch Serverless collection, observing whether the configured index exists before any event is sent. Compare this behavior with non-serverless OpenSearch sinks and identify the existing startup index-creation tests or entry points. Done means the configured index is created when the pipeline starts, before documents arrive.