Merge pull request #228 from antonyrag/release/ragleap-graph-v0.6.8

[0.6.8]

Fixed

• CO_OCCURS_WITH relationship-MERGE race, noticed but deliberately left open in the v0.6.7 fix (that release closed the four NODE-level races for Document/Entity/PairWeight/RelationWeight; this closes the equivalent race at the relationship level). MERGE (ea)-[r:CO_OCCURS_WITH]-(eb) matched only on the two Entity endpoints and relationship type - not atomic against concurrent writers, same class of bug as the v0.6.7 node-level races.
• Fix: a composite_key property (same _composite_key() helper as the four node types) is now MERGE'd on the CO_OCCURS_WITH relationship, backed by a relationship-level uniqueness constraint (FOR ()-[r:CO_OCCURS_WITH]-() REQUIRE r.composite_key IS UNIQUE), created idempotently alongside the existing four node constraints. CO_OCCURS_WITH is undirected and entity-pair order wasn't canonicalized anywhere in the existing code, so the composite_key is computed from a sorted (a, b) pair - without this, the same logical entity pair extracted in different orders across different documents would compute two different keys and MERGE could still create a duplicate relationship.
• Added backfill_co_occurs_with_composite_key(namespace=None, batch_size=500) - the same one-time, idempotent migration pattern as backfill_composite_key() (v0.6.7), for CO_OCCURS_WITH relationships written before this version. Deduplicates on relationship identity (WITH DISTINCT r) before reading endpoint names via startNode(r)/endNode(r), since Neo4j's undirected-match semantics can otherwise surface the same relationship via more than one traversal direction.

Verified

• Live concurrency test (8 concurrent upsert_document() calls, 8 different document_ids, all mentioning the same two entities) against real Neo4j: exactly 1 CO_OCCURS_WITH relationship resulted, with composite_key correctly set - confirming the fix under real contention, not just code review. This is a different race shape than v0.6.7's Document-race test (same document_id, concurrent calls), since CO_OCCURS_WITH deliberately aggregates across DIFFERENT documents by design.
• Full suite: 79 passed, 12 skipped - zero regressions.