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_test.go

@@ -234,6 +234,60 @@ func TestImageClientSendsReferenceImagesByURLWhenConfigured(t *testing.T) {
 	}
 }
 
+func TestImageClientUploadsLocalhostReferenceImagesEvenWhenURLTransportConfigured(t *testing.T) {
+	var sawMultipartEdit bool
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/reference.png":
+			w.Header().Set("Content-Type", "image/png")
+			_, _ = w.Write([]byte("local image bytes"))
+		case "/v1/images/edits":
+			sawMultipartEdit = true
+			if !strings.HasPrefix(r.Header.Get("Content-Type"), "multipart/form-data") {
+				t.Errorf("expected multipart edit request for local image, got %s", r.Header.Get("Content-Type"))
+			}
+			if err := r.ParseMultipartForm(32 << 20); err != nil {
+				t.Errorf("parse multipart form: %v", err)
+				return
+			}
+			file, header, err := r.FormFile("image")
+			if err != nil {
+				t.Errorf("expected image file field: %v", err)
+				return
+			}
+			defer file.Close()
+			data, err := io.ReadAll(file)
+			if err != nil {
+				t.Errorf("read image file: %v", err)
+				return
+			}
+			if header.Filename != "reference.png" || header.Header.Get("Content-Type") != "image/png" || string(data) != "local image bytes" {
+				t.Errorf("unexpected uploaded image: filename=%q content_type=%q data=%q", header.Filename, header.Header.Get("Content-Type"), string(data))
+			}
+			w.Header().Set("Content-Type", "application/json")
+			_, _ = w.Write([]byte(`{"data":[{"url":"https://example.com/edited.png"}]}`))
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer server.Close()
+
+	client := NewImageClient(ImageClientOptions{BaseURL: server.URL, APIKey: "test-key", InputImageTransport: "url"})
+	image, err := client.Generate(context.Background(), "edit the local image", ImageRequestOptions{
+		Model:  "gpt-image-2",
+		Images: []string{server.URL + "/reference.png"},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if image.URL != "https://example.com/edited.png" {
+		t.Fatalf("expected edited image, got %#v", image)
+	}
+	if !sawMultipartEdit {
+		t.Fatal("expected multipart image edit request")
+	}
+}
+
 func TestImageClientUploadsInternalReferenceImagesAsMultipart(t *testing.T) {
 	var sawEdit bool
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {