fix(piagent): upload local and private reference images as multipart

When URL input transport is configured, the remote image API cannot fetch
localhost or private-network reference URLs. Detect such hosts and fall back
to multipart file upload so those references still reach the model.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/infrastructure/piagent/image_generation.go

@@ -21,6 +21,7 @@ import (
 	"unicode/utf8"
 
 	"img_infinite_canvas/internal/domain/design"
+	"img_infinite_canvas/internal/infrastructure/netutil"
 
 	"github.com/zeromicro/go-zero/core/logx"
 )
@@ -219,7 +220,7 @@ func (c *ImageClient) Generate(ctx context.Context, prompt string, opts ImageReq
 
 func (c *ImageClient) imageRequestBody(ctx context.Context, model string, prompt string, size string, count int, images []string, stream bool) ([]byte, string, error) {
 	if len(images) > 0 {
-		if c.inputImageTransport == "url" {
+		if c.inputImageTransport == "url" && imageInputsCanUseURLTransport(images) {
 			return jsonImageEditBody(model, prompt, size, count, images, stream)
 		}
 		return c.multipartImageEditBody(ctx, model, prompt, size, count, images, stream)
@@ -242,6 +243,38 @@ func (c *ImageClient) imageRequestBody(ctx context.Context, model string, prompt
 	return body, "application/json", err
 }
 
+func imageInputsCanUseURLTransport(images []string) bool {
+	for _, image := range images {
+		if imageInputRequiresFileUpload(image) {
+			return false
+		}
+	}
+	return true
+}
+
+func imageInputRequiresFileUpload(value string) bool {
+	value = strings.TrimSpace(value)
+	if value == "" {
+		return false
+	}
+	if strings.HasPrefix(value, "data:image/") || strings.HasPrefix(value, "/") {
+		return true
+	}
+	parsed, err := url.Parse(value)
+	if err != nil {
+		return true
+	}
+	scheme := strings.ToLower(parsed.Scheme)
+	if scheme != "http" && scheme != "https" {
+		return true
+	}
+	host := strings.ToLower(parsed.Hostname())
+	if host == "" {
+		return true
+	}
+	return netutil.IsLocalOrPrivateHost(host)
+}
+
 func jsonImageEditBody(model string, prompt string, size string, count int, images []string, stream bool) ([]byte, string, error) {
 	requestPayload := map[string]any{
 		"model":  model,