feat: opensearch add asynchronous similarity search methods and refac…

[miguelgonzpk]

PR Title:
feat(community): add native async support to OpenSearchVectorSearch

PR Message:

Description:
This PR implements native asynchronous support for the OpenSearchVectorSearch vector store in langchain-community, replacing the default run_in_executor workaround with true non-blocking calls using the AsyncOpenSearch client.

Key changes:

• Implemented asimilarity_search, asimilarity_search_with_score, and asimilarity_search_with_score_by_vector using self.async_client.
• Added _araw_similarity_search_with_score_by_vector to handle the low-level async API calls, maintaining feature parity with the synchronous version (including hybrid_search, script_scoring, painless_scripting, and routing support).
• Refactored the response parsing logic into a new private helper method _hits_to_documents to reuse code between sync and async implementations (DRY).
• Fixed an issue where routing parameters might be ignored in async contexts.

Issue:
N/A

Dependencies:
No new dependencies (relies on existing opensearch-py).

• Lint and test: I have run make format, make lint and make test from the root of the package(s).