test: add tests for network_policy.get_policy_engine singleton (closes #2445)

[tmdeveloper007]

Summary

Adds unit tests for backend.secuscan.network_policy.get_policy_engine,a module-level singleton accessor.

Changes

• New test file: testing/backend/unit/test_network_policy_engine_singleton.py
• 6 tests covering: first-call instance creation, singleton return,
  engine public methods, persistence across audit clears,
  and correct settings initialization

Testing

• All 6 tests pass with python -m pytest testing/backend/unit/test_network_policy_engine_singleton.py --noconftest -v

[utksh1]

Closing this pull request because it bundles two unrelated changes under the #2445 issue and duplicates the focused submissions #2454 (network-policy singleton tests) and #2449 (parser sandbox capability tests). Splitting the work keeps review and attribution clear.