feat(elasticsearch): add ElasticsearchInferenceDenseRetriever

[chakshu-dhannawat]

Closes #2945.

Summary

Adds server-side dense retrieval via Elasticsearch's inference API — no local embedding model required. The query vector is generated by an Elasticsearch inference endpoint at search time using knn.query_vector_builder.text_embedding.

This completes the inference retriever family alongside the existing ElasticsearchInferenceSparseRetriever and ElasticsearchInferenceHybridRetriever.

Changes

ElasticsearchDocumentStore — three new methods:

• _create_dense_retrieval_inference_body: builds the kNN search body with query_vector_builder (instead of a pre-computed query_vector), with optional filters and num_candidates
• _dense_retrieval_inference: sync retrieval
• _dense_retrieval_inference_async: async retrieval

ElasticsearchInferenceDenseRetriever — new @component:

• __init__: document_store, inference_id, filters, top_k, num_candidates, filter_policy
• run / run_async: sync and async retrieval
• to_dict / from_dict: full serialization support
• Exported from haystack_integrations.components.retrievers.elasticsearch

tests/test_cloud_dense_retriever.py:

• Unit tests: init defaults/custom, validation errors, serde round-trip, run/run_async, filter policies, body shape (query_vector_builder present, query_vector absent), edge cases
• Integration test class (@pytest.mark.integration) — skipped unless ELASTICSEARCH_URL, ELASTIC_API_KEY, and ELASTICSEARCH_DENSE_INFERENCE_ID are set

Usage

from haystack_integrations.document_stores.elasticsearch import ElasticsearchDocumentStore
from haystack_integrations.components.retrievers.elasticsearch import ElasticsearchInferenceDenseRetriever

document_store = ElasticsearchDocumentStore(
    hosts="https://my-cluster.es.io:443",
    api_key="...",
)
retriever = ElasticsearchInferenceDenseRetriever(
    document_store=document_store,
    inference_id=".multilingual-e5-small-elasticsearch",  # built-in on Serverless
)
result = retriever.run(query="Find documents about Paris")

[github-actions]

Coverage report (elasticsearch)

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
integrations/elasticsearch/src/haystack_integrations/components/retrievers/elasticsearch
inference_dense_retriever.py
integrations/elasticsearch/src/haystack_integrations/document_stores/elasticsearch
document_store.py					1619-1626, 1647-1655
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