test : add unit tests for _gather_scan_summary in notification_service

[tmdeveloper007]

Summary of What Needs to be Done:
Add unit tests for the _gather_scan_summary() async helper function in backend/secuscan/notification_service.py. This function collects task status, severity counts, and a report link for a scan completion webhook. It queries the database for task and finding records.

The existing test file testing/backend/unit/test_notification_service.py has 36 tests that cover process_scan_completion_webhook and other delivery functions, but _gather_scan_summary is not directly imported or tested — it is only exercised indirectly through process_scan_completion_webhook.

Changes that Need to be Made:
Add a new test section (or new test class) in testing/backend/unit/test_notification_service.py:

• Import _gather_scan_summary from backend.secuscan.notification_service
• Test that it returns None when the task_id does not exist in the database
• Test that it returns a summary dict with correct structure when the task exists
• Test that severity_counts correctly aggregates findings by severity
• Test that the report_link includes the task_id and uses the configured base_url
• Test that error_message from the task record is included in the summary
• Test that tool_name falls back to plugin_id when tool_name is not set
• Use the existing test_db fixture (AsyncDatabase) to set up test data

Impact that it would Provide:

• Directly validates the summary aggregation logic independent of webhook delivery
• Ensures severity counts are computed correctly regardless of webhook configuration
• Provides clear unit-level coverage for the helper before it is used in more complex flows

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