Do not update notification to sending if the status is already final

This prevents a race condition when we get delivery receipt before
updating notification to sending, and so the sending status would
supersede the delivered status, and the notification would time out
as temporary-failure after three days.

tests/app/delivery/test_send_to_providers.py

@@ -9,7 +9,7 @@ from notifications_utils.recipients import validate_and_format_phone_number
 from requests import HTTPError
 
 import app
-from app import mmg_client, firetext_client
+from app import clients, mmg_client, firetext_client
 from app.dao import notifications_dao
 from app.dao.provider_details_dao import get_provider_details_by_identifier
 from app.delivery import send_to_providers
@@ -525,6 +525,20 @@ def test_should_not_update_notification_if_research_mode_on_exception(
     assert update_mock.called
 
 
+@pytest.mark.parametrize("starting_status, expected_status", [
+    ("delivered", "delivered"),
+    ("created", "sending"),
+    ("technical-failure", "technical-failure"),
+])
+def test_update_notification_to_sending_does_not_update_status_from_a_final_status(
+    sample_service, notify_db_session, starting_status, expected_status
+):
+    template = create_template(sample_service)
+    notification = create_notification(template=template, status=starting_status)
+    send_to_providers.update_notification_to_sending(notification, clients.get_client_by_name_and_type("mmg", "sms"))
+    assert notification.status == expected_status
+
+
 def __update_notification(notification_to_update, research_mode, expected_status):
     if research_mode or notification_to_update.key_type == KEY_TYPE_TEST:
         notification_to_update.status = expected_status