[Feature]: Minimal tests from backup PR (relational helpers only)

[Vasilije1990]

Problem Statement

Goal: Backport the smallest tests from the backup PR that validate the four helpers (no orchestration/adapters).

Proposed Solution

Scope: Tests that seed Node/Edge rows and assert dataset/data-scoped deletions.
Files (example destinations):
cognee/tests/unit/graph/test_delete_dataset_related_nodes_edges.py
cognee/tests/unit/graph/test_delete_data_related_nodes_edges.py
Acceptance Criteria:
Tests pass locally and in CI with no additional file changes.
No integration tests or engine calls included here.

Alternatives Considered

No response

Use Case

Delete functionality

Implementation Ideas

No response

Additional Context

No response

Pre-submission Checklist

• I have searched existing issues to ensure this feature hasn't been requested already
• I have provided a clear problem statement and proposed solution
• I have described my specific use case