fix(lint): check WriteString return value in summarizeOpenAIImagesNoOutputBody

golangci-lint (errcheck) flagged the unchecked (*strings.Builder).WriteString
return value at openai_images_responses.go:632. Align with the existing
convention used elsewhere in the package (e.g. gateway_service.go) by
explicitly discarding the return values with `_, _ =`.

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)
 	}