test : added unit tests for build_alert_payload in notification_service

[tmdeveloper007]

Closes #1705.

Summary of What Has Been Done:
Added testing/backend/unit/test_notification_service_build_alert_payload.py with 8 unit tests covering build_alert_payload in backend/secuscan/notification_service.py.

Changes Made:

• test_builds_payload_with_all_fields: verifies complete finding and rule dicts produce the correct payload structure with event, rule, and finding sections
• test_missing_metadata_json_produces_empty_metadata: missing metadata_json key results in metadata={}
• test_invalid_metadata_json_falls_back_to_raw_string: non-JSON metadata_json is caught by the try/except and stored as {"raw": "..."}
• test_non_string_metadata_json_raises_type_error_and_falls_back: int metadata_json raises TypeError caught by the except clause
• test_missing_finding_fields_produce_none_in_payload: optional finding fields that are missing appear as None in the payload
• test_valid_dict_metadata_json_is_parsed_and_redacted: valid JSON dict in metadata_json is parsed and redact_inputs applied
• test_rule_channel_type_is_included: rule.channel_type is present in the payload rule section
• test_redact_dict_is_called_on_result: mocks redact_dict to verify it is called on the final payload

Impact it Made:

• Brings build_alert_payload under test coverage for the first time
• Tests all error paths: missing keys, invalid JSON, wrong types
• Verifies the redaction pipeline is applied to outbound alert payloads

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