Auto-Tangents node with spread = 0 and preserve existing = true causes weird change

When spread = 0 and preserve existing = true, we would expect the node to have no effect. But instead, it makes a weird slight change to the geometry based on doing something to the handles as we see here, which goes away when spread becomes slightly larger than 0:

https://github.com/user-attachments/assets/1a418c65-20c5-4e1a-b01e-43a5ad68a1d6

Paste this into a document:
```
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```

Tái hiện vấn đề bằng tài liệu đã được tuần tự hóa trong báo cáo, với nút Auto-Tangents được đặt thành spread = 0 và preserve existing = true. So sánh hình học với cùng tài liệu nhưng có spread lớn hơn một chút, sau đó tìm phần triển khai Auto-Tangents và các bài kiểm thử hiện có. Hoàn tất khi spread = 0 giữ nguyên hình học.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.