feat(elasticsearch): add close(), aclose(), and context manager support to ElasticsearchDocumentStore

[chakshu-dhannawat]

Summary

Closes #1628.

ElasticsearchDocumentStore holds open HTTP connection pools (urllib3 for the sync Elasticsearch client, aiohttp for the async AsyncElasticsearch client). Without explicit cleanup, connections leak in long-running services and test suites — especially the async transport which owns an aiohttp.ClientSession.

This PR adds:

Method	Description
close()	Close both clients synchronously
aclose()	Close async client with await, sync client inline
__enter__ / __exit__	Sync context manager (calls close)
__aenter__ / __aexit__	Async context manager (calls aclose)

All four are no-ops when called before initialization (clients are None).

Usage after this PR:

# sync context manager
with ElasticsearchDocumentStore(hosts="https://localhost:9200") as store:
    store.write_documents(docs)

# async context manager
async with ElasticsearchDocumentStore(hosts="https://localhost:9200") as store:
    await store.write_documents_async(docs)

# explicit close
store = ElasticsearchDocumentStore(hosts="https://localhost:9200")
try:
    store.write_documents(docs)
finally:
    store.close()

Test plan

Three new unit tests (no ES cluster needed):

• test_close_calls_client_close — verifies close() calls .close() on both mock clients
• test_close_before_init_is_safe — verifies no error when closing before initialization
• test_context_manager_calls_close — verifies __exit__ calls close via the sync context manager

[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/document_stores/elasticsearch
document_store.py					326-329, 338, 341
Project Total

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