Pagination fields are missing from assistants list beta API (#571)

curl "https://api.openai.com/v1/assistants?order=desc&limit=20" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $OPENAI_API_KEY" \
  -H "OpenAI-Beta: assistants=v1"
{
  "object": "list",
  "data": [],
  "first_id": null,
  "last_id": null,
  "has_more": false
}

assistant_test.go

@@ -142,6 +142,8 @@ When asked a question, write and run Python code to answer the question.`
 				fmt.Fprintln(w, string(resBytes))
 			} else if r.Method == http.MethodGet {
 				resBytes, _ := json.Marshal(openai.AssistantsList{
+					LastID:  &assistantID,
+					FirstID: &assistantID,
 					Assistants: []openai.Assistant{
 						{
 							ID:           assistantID,