test(responses): 模型改写转发测试夹具补齐上游 created_at

网关现会为缺 created_at 的 Response 对象补齐该字段（#8118），该测试以
"输出包含上游原始字节"断言模型改写，夹具缺 created_at 时输出多出补齐字段
而失配。夹具改为真实上游必带 created_at 的形态，测试意图（仅改写 model）不变。

backend/internal/service/openai_response_model_rewrite_test.go

@@ -56,7 +56,7 @@ func TestMappedResponseModelForwarding(t *testing.T) {
 						contentType := "application/json"
 						body := payload
 						if kind == "responses" {
-							payload = `{"type":"response.completed","response":{"id":"resp_1","model":"` + returned + `","output":[],"usage":{"input_tokens":1,"output_tokens":1}}}`
+							payload = `{"type":"response.completed","response":{"id":"resp_1","created_at":1700000000,"model":"` + returned + `","output":[],"usage":{"input_tokens":1,"output_tokens":1}}}`
 							want = strings.Replace(payload, `"model":"`+returned+`"`, `"model":"`+model+`"`, 1)
 						}
 						if kind != "json" {