Refactor similarity_search function in elastic_vector_search.py (#2761)

Optimization :Limit search results when k < 10
Fix issue when k > 10: Elasticsearch will return only 10 docs


[default-search-result](https://www.elastic.co/guide/en/elasticsearch/reference/current/paginate-search-results.html)
By default, searches return the top 10 matching hits

Add size parameter to the search request to limit the number of returned
results from Elasticsearch. Remove slicing of the hits list, since the
response will already contain the desired number of results.

langchain/vectorstores/elastic_vector_search.py

@@ -200,8 +200,8 @@ class ElasticVectorSearch(VectorStore, ABC):
         """
         embedding = self.embedding.embed_query(query)
         script_query = _default_script_query(embedding)
-        response = self.client.search(index=self.index_name, query=script_query)
-        hits = [hit["_source"] for hit in response["hits"]["hits"][:k]]
+        response = self.client.search(index=self.index_name, query=script_query, size=k)
+        hits = [hit["_source"] for hit in response["hits"]["hits"]]
         documents = [
             Document(page_content=hit["text"], metadata=hit["metadata"]) for hit in hits
         ]