test : add unit tests for build_alert_payload in notification_service

[tmdeveloper007]

Summary of What Needs to be Done:

Add unit tests for build_alert_payload in backend/secuscan/notification_service.py. This function builds a redacted JSON-serializable alert payload from a finding and notification rule dict. It currently has no direct unit tests.

The function:

1. Extracts and parses metadata_json from the finding dict (redacts on parse failure)
2. Builds a structured payload with rule metadata and finding fields
3. Applies redact_dict to the final payload before returning

Changes that Need to be Made:

Create testing/backend/unit/test_notification_service_build_alert_payload.py with the following test cases:

• test_builds_payload_with_all_fields: finding and rule dicts with all standard keys produce expected payload shape and values
• test_missing_metadata_json_produces_empty_metadata: finding without metadata_json key results in metadata={}
• test_invalid_metadata_json_falls_back_to_raw_string: metadata_json="not-json" is caught by try/except and stored as {"raw": "not-json"}
• test_non_string_metadata_json_raises_type_error_and_falls_back: metadata_json=123 raises TypeError caught by the except clause
• test_missing_finding_fields_produce_none_in_payload: missing task_id, plugin_id, description etc. appear as None in payload
• test_redact_dict_is_called_on_result: the returned dict should have no plain-text secrets — patch redact_dict and verify it is called once
• test_rule_channel_type_is_included: verify rule.channel_type is present in the payload rule section

Import build_alert_payload from backend.secuscan.notification_service. The redact_dict and redact_inputs imports from the same module should be patched in the appropriate test.

Impact that it would Provide:

• Reliability: ensures alert payloads are correctly structured for webhook/email delivery
• Regression safety: prevents schema breakage when finding/rule field names change
• Coverage: the function handles error paths (bad JSON, missing fields) that are not exercised anywhere else

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