[BUG] Index prefix parsing bug in OpenSearch sink

Describe the bug
Index prefix is not parsed correctly when the index name contains dynamic date pattern and the date pattern is not prefixed or suffixed with a hypen, for example, if index is configured as index-prefix.%{yyyy-MM-dd}, the parsed index prefix will be index-prefix.%{yyyy-MM-dd} instead of index-prefix.

This bug can manifest itself as a misleading [security_exception] authentication/authorization failure when pipeline tries to check if a corresponding index template exists. See details below.

To Reproduce
Steps to reproduce the behavior:

1. Configure a pipeline with this opensearch sink config pointing to a public AOSS collection:

  sink:
    - opensearch:
        hosts:
          - https://xxxx.us-east-1.aoss.amazonaws.com
        index_type: custom
        index: "prefix.%{yyyy-MM-dd}"
        aws:
          sts_role_arn: arn:aws:iam::xxx:role/OpenSearchServerlessPipelineRole
          region: us-east-1
          serverless: true
        template_type: index-template
        template_content: '{"template":{"mappings":{"date_detection":false}}}'

2. Start the pipeline, pipeline fails to init sink with this error message:

org.opensearch.client.opensearch._types.OpenSearchException: Request failed: [security_exception] authentication/authorization failure
        at org.opensearch.client.transport.aws.AwsSdk2Transport.parseResponse(AwsSdk2Transport.java:473) ~[opensearch-java-2.8.1.jar:?]
        at org.opensearch.client.transport.aws.AwsSdk2Transport.executeSync(AwsSdk2Transport.java:392) ~[opensearch-java-2.8.1.jar:?]
        at org.opensearch.client.transport.aws.AwsSdk2Transport.performRequest(AwsSdk2Transport.java:192) ~[opensearch-java-2.8.1.jar:?]
        at org.opensearch.client.opensearch.indices.OpenSearchIndicesClient.existsIndexTemplate(OpenSearchIndicesClient.java:571) ~[opensearch-java-2.8.1.jar:?]
        at org.opensearch.dataprepper.plugins.sink.opensearch.index.ComposableTemplateAPIWrapper.getTemplate(ComposableTemplateAPIWrapper.java:45) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
        at org.opensearch.dataprepper.plugins.sink.opensearch.index.ComposableIndexTemplateStrategy.getExistingTemplateVersion(ComposableIndexTemplateStrategy.java:28) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
        at org.opensearch.dataprepper.plugins.sink.opensearch.index.AbstractIndexManager.shouldCreateTemplate(AbstractIndexManager.java:292) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
        at org.opensearch.dataprepper.plugins.sink.opensearch.index.AbstractIndexManager.checkAndCreateIndexTemplate(AbstractIndexManager.java:246) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
        at org.opensearch.dataprepper.plugins.sink.opensearch.index.AbstractIndexManager.checkAndCreateIndexTemplate(AbstractIndexManager.java:234) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
        at org.opensearch.dataprepper.plugins.sink.opensearch.index.AbstractIndexManager.setupIndex(AbstractIndexManager.java:224) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
        at org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink.doInitializeInternal(OpenSearchSink.java:235) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
        at org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink.doInitialize(OpenSearchSink.java:193) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
        at org.opensearch.dataprepper.model.sink.AbstractSink.initialize(AbstractSink.java:52) ~[data-prepper-api-2.7.0-SNAPSHOT.jar:?]
...

Expected behavior
Sink should initialize correctly.

Screenshots
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Environment (please complete the following information):

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

Additional context
Relevant code:
https://github.com/opensearch-project/data-prepper/blob/1e5e0d06b825923ccce47689d3cbf5bccd365a29/data-prepper-plugins/opensearch/src/main/java/org/opensearch/dataprepper/plugins/sink/opensearch/index/AbstractIndexManager.java#L127-L129