RunRetrieve - add RunStatus.Incomplete

REF: https://platform.openai.com/docs/assistants/how-it-works/managing-threads-and-messages
The "run step object" documentation doesn't mention this as a possible status, but it is referenced at the above page, and I have observed it is used

OpenAI.SDK/ObjectModels/StaticValueHelper.cs

@@ -155,6 +155,7 @@ public static class RunStatus
             public static string Failed => "failed";
             public static string Completed => "completed";
             public static string Expired => "expired";
+            public static string Incomplete => "incomplete";
         }
 
         public static class RequiredActionTypes