test : added unit tests for _gather_scan_summary in notification_service

[tmdeveloper007]

Closes #2423.

Summary of What Has Been Done:
Created a new test file testing/backend/unit/test_notification_service_gather_scan_summary.py with 8 unit tests for the _gather_scan_summary() async helper in backend/secuscan/notification_service.py.

The function is exercised indirectly through process_scan_completion_webhook but not directly tested.

Changes Made:

• New file: testing/backend/unit/test_notification_service_gather_scan_summary.py
• Inline setup_test_environment fixture (to avoid loading FastAPI-dependent conftest)
• Inline _seed_task_with_findings helper for test data setup
• Added 8 tests covering:
  • Returns None for nonexistent task_id
  • Returns correct dict structure with all expected keys
  • Severity counts correctly aggregated (2 critical, 1 high, 1 low)
  • Zero findings gives total_findings=0 and empty counts
  • error_message from task included in summary
  • report_link contains the task_id
  • status is lowercased
  • tool_name uses the database value

Impact it Made:

• Directly validates the summary aggregation logic independent of webhook delivery
• Ensures severity counts are computed correctly
• Provides clear unit-level coverage for the helper

Note: This task is being handled by tmdeveloper007 — please assign to that account when picking it up.

[utksh1]

The code change is reviewed and the backend/lint checks are green. Merge remains blocked by the required frontend audit, which reports the pre-existing high-severity undici vulnerability; I am not bypassing that security gate.