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

### 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_