[FEATURE] Improve test coverage: storage error handling and multi-tenant isolation

[mykola-pereyma]

Package

lexical-graph

Problem statement

OpenSearch error handling has minimal test coverage (only NotFoundError retry tested). Neptune error tests already exist in test_neptune_graph_stores.py. Checkpoint, cascading deletion, and multi-tenant vector store have no error-path tests.

Related: #408

Proposed solution

• OpenSearch: connection timeout, bulk insert partial failure, auth failure (403), index_exists 404 handling
• Checkpoint: missing directory, unreadable file (PermissionError), read-only filesystem
• delete_sources: error mid-cascade propagation, empty source_ids list
• Multi-tenant: query/write with wrong tenant scoped correctly, no cross-tenant leakage

Alternatives considered

No response