[PM-23249] Prevent log-out when changing KDF settings (#6349)

* Prevent log-out when changing KDF settings with feature flag.

* validate salt unchanged for user to throw bad request (400), not internal server error (500)

* change kdf integration tests

* failing tests

* iuncorrect tests wording

* conditional logout

* log out reason as enum

* explicit naming

src/Notifications/HubHelpers.cs

@@ -64,7 +64,7 @@ public static class HubHelpers
             case PushType.SyncSettings:
             case PushType.LogOut:
                 var userNotification =
-                    JsonSerializer.Deserialize<PushNotificationData<UserPushNotification>>(
+                    JsonSerializer.Deserialize<PushNotificationData<LogOutPushNotification>>(
                         notificationJson, _deserializerOptions);
                 await hubContext.Clients.User(userNotification.Payload.UserId.ToString())
                     .SendAsync(_receiveMessageMethod, userNotification, cancellationToken);