lightrag-memgraph: get_docs_paginated() unstable pagination (no secondary sort key)

[antejavor]

What's wrong

MemgraphDocStatusStorage.get_docs_paginated() in integrations/lightrag-memgraph/src/lightrag_memgraph/docstatus_impl.py sorts by a single field with no secondary tiebreaker:

ORDER BY {sort_expr} {order}

The reference JsonDocStatusStorage sorts an in-memory Python list, whose sort is stable and gives deterministic tie-breaking (by insertion order) for free. Memgraph's ORDER BY gives no stability guarantee among tied rows across separate query executions.

LightRAG commonly batch-writes many documents sharing the identical updated_at/created_at timestamp within one ingestion batch. Paging through the same listing (page 1, then page 2) can return the same document twice or silently skip one whenever ties exist — a pagination-correctness regression from the reference semantics. Doesn't show up in the current single-page integration tests, only with real timestamp-colliding data.

Acceptance criteria

• get_docs_paginated()'s ORDER BY includes a secondary, always-unique tiebreaker (e.g. id)
• Test with multiple documents sharing the same sort-field value, asserting stable results across two paginated calls

Blocked by

None - can start immediately