Include index name in OpenSearchVectorStore for similarity search

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Bug description When performing a similarity search with SearchRequest, the index name is not included. This leads to a permission error when using roles restricted to specific indices in Opensearch.

Contents

• Fixed the issue where the index name was missing in similaritySearch. The index was included for adding and deleting documents but not for search. I added it to ensure correct behavior.

Test

• Added tests to verify documents are correctly added and retrieved.
• Tested this with different indices using separate OpenSearchVectorStore instances.

Issue Link close #885