fix(vector_stores/elasticsearch): stop reading transport failures as missing vectors

[siye566]

{"body": "## What & Why\n\nElasticsearchDB.get() wrapped the client call and response parsing in one try whose blanket except Exception returned None for everything \u2014 including connection refusals, auth failures and cluster outages. None is what every caller reads as "no such vector", so a backend outage looked like missing data. The codebase already documents the opposite contract: _update_memory() in mem0/memory/main.py wraps vector_store.get() in except Exception: ... raise with the comment "Backing-store failure, not a bad memory_id: re-raise the original so the REST layer maps it to 5xx, not 4xx". The update() path is worse: None there inserts a duplicate row where the user asked for an update (#7516).\n\nFix: split get() into two phases \u2014\n\n- NotFoundError from the client call (a missing document, the only genuine "no such vector") returns None;\n- every other client-call failure logs and re-raises;\n- response parse-shape failures (KeyError/TypeError on _id/_source) keep degrading to None.\n\n## Tests\n\n- test_get_not_found now uses the real NotFoundError (it previously pinned the bug with a generic Exception(\"Not found\") \u2192 None).\n- New test_get_transport_error_propagates: a ConnectionError propagates (verified red on the old code).\n- New test_get_malformed_response_returns_none: missing _source and a None response still degrade to None.\n\nFull tests/vector_stores/test_elasticsearch.py: 24 passed (Python 3.12, elasticsearch 8.19.3). ruff check / ruff format --check clean.\n\nFixes #7516"}

[github-actions]

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[github-actions]

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