fix: align Neptune community AOSS index

[iroiro147]

Summary

• align the shared COMMUNITY_INDEX_NAME default with Neptune's registered AOSS community index (community_name)
• use that same constant for AOSS registration, Neptune community saves, fallback CommunityNode.save, and Neptune community fulltext search
• remove the invalid await around the sync Neptune save_to_aoss fallback call
• add focused regression coverage for the community AOSS index contract

Closes #1632

Testing

• make format
• make lint
• uv run pytest tests/driver/test_neptune_aoss_indices.py tests/driver/test_falkordb_driver.py -q -> 28 passed, 1 skipped, 1 warning
• UV_CACHE_DIR=/tmp/uv-cache uv run pytest tests/driver/test_neptune_aoss_indices.py -q -> 3 passed, 1 warning

Notes

• I also attempted make test; it reached 168 passed, 2 skipped, 10 deselected, then errored on unavailable local Neo4j at localhost:7687 in tests/test_add_triplet.py. I interrupted the repeated Neo4j retries after confirming the failure was external-service setup, not this candidate change.

[iroiro147]

Hi maintainers — a gentle follow-up on this bug fix for #1632. It aligns the Neptune/AOSS community index name across registration, save, fallback, and search paths, removes the invalid await in the fallback, and includes focused regression coverage. GitHub reports the branch mergeable; CLA, Ruff, and fork checks pass, and the remaining triage failure is the repository automation’s external-contributor permission check. Happy to adjust the implementation or tests if you would prefer a different approach.