test : added unit tests for get_delivery_configuration in notification_service

[tmdeveloper007]

Closes #1706.

Summary of What Has Been Done:
Added testing/backend/unit/test_notification_service_delivery_config.py with 7 unit tests covering get_delivery_configuration in backend/secuscan/notification_service.py.

Changes Made:

• test_returns_expected_keys: verifies all 4 required keys are present
• test_webhook_timeout_is_positive_number: webhook_timeout_seconds is a positive number
• test_webhook_connect_timeout_is_positive_number: webhook_connect_timeout_seconds is a positive number
• test_max_retries_is_zero: max_retries equals 0
• test_backoff_factor_is_zero: backoff_factor_seconds equals 0.0
• test_returns_new_dict_each_call: each call returns an independent dict (no shared mutable state)
• test_timeout_values_are_reasonable: timeout values are < 300s

Impact it Made:

• Brings the static delivery configuration function under test coverage
• Prevents accidental changes to critical timeout and retry values
• Documents the expected shape of the delivery configuration

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