fix(lint): check WriteString return in summarizeOpenAIImagesNoOutputBody

修复 PR #3381 引入的 errcheck lint 错误，satisfy golangci-lint
对 strings.Builder.WriteString 返回值的检查要求。

backend/internal/service/openai_images_responses.go

@@ -629,7 +629,7 @@ func summarizeOpenAIImagesNoOutputBody(body []byte) string {
 		}
 	})
 	var b strings.Builder
-	b.WriteString("no_image_output")
+	_, _ = b.WriteString("no_image_output")
 	if lastType != "" {
 		fmt.Fprintf(&b, " last_event=%s", lastType)
 	}