fix(canvas): 画布智能体 - 严格校验模型选择并修正任务与 DNS 错误诊断

backend/internal/handler/openapi.yaml

@@ -1203,6 +1203,7 @@ components:
                 - timeout
                 - internal
                 - bad_gateway
+                - upstream_dns_failed
                 - model_capability_not_supported
                 - model_price_not_configured
                 - model_route_unavailable

backend/internal/handler/response_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"strings"
 	"testing"
 
@@ -40,6 +41,20 @@ func TestFailServiceProjectsAppError(t *testing.T) {
 	}
 }
 
+func TestFailServiceProjectsDNSFailureWithoutTransportDetails(t *testing.T) {
+	recorder, context := responseTestContext()
+	err := service.WrapAppError(http.StatusBadGateway, "外部服务域名解析失败，请检查渠道域名和后端 DNS 配置", errors.New("private-sentinel resolver failure"))
+	err.Reason = service.ReasonUpstreamDNSFailed
+	failService(context, &url.Error{Op: "Post", URL: "https://private-sentinel.invalid?token=private-sentinel", Err: err})
+	response := decodeFailureEnvelope(t, recorder)
+	if recorder.Code != http.StatusBadGateway || response.Code != service.CodeBadGateway || response.Reason != string(service.ReasonUpstreamDNSFailed) || response.Msg != err.Message {
+		t.Fatalf("DNS response contract lost: %d %#v", recorder.Code, response)
+	}
+	if strings.Contains(recorder.Body.String(), "private-sentinel") {
+		t.Fatal("raw transport details leaked into HTTP response")
+	}
+}
+
 func TestFailServiceQuotaExceeded(t *testing.T) {
 	recorder, context := responseTestContext()
 	failService(context, service.QuotaExceeded("账号素材数量已达到 100 个上限"))