Path tool creates unexpected 'Flatten Path' node

This issue results in an error in the node graph, causing the document to not be rendered.

The *Flatten Path* node is created in utility_types.rs: https://github.com/GraphiteEditor/Graphite/pull/2228/files#diff-f9c3a49009b09a2d6bd1ebb8b090622e4a0e9c19f760f501d006bdb09e42759dR299

This was initially caused by creating a single file and using the path tool on an ellipse (I had two on the canvas), but it was difficult to reproduce.

To consistently reproduce, open these three files ([error_1-and-3.graphite.txt](https://github.com/user-attachments/files/18559523/error_1-and-3.graphite.txt) and [error_2.graphite.txt](https://github.com/user-attachments/files/18559524/error_2.graphite.txt) are the two files needed) in succession on a clean Graphite instance, using `File > Open...`, in the given order:

`error_1-and-3.graphite.txt`, `error_2.graphite.txt`, and finally `error_1-and-3.graphite.txt`.

Then, in the last file (the one currently open), use the path tool to drag a point of the black ellipse (the other layers/shapes are extraneous data relative to the error).

> #2214 is a great example of the kind of situation we're looking to get bug reports for

Discord discussion: https://discord.com/channels/731730685944922173/860830913390575646/1333447293021720706

Bắt đầu trong utility_types.rs, tại phần tạo nút Flatten Path được tham chiếu trong issue. Tái hiện trên một phiên bản Graphite sạch bằng cách mở error_1-and-3.graphite.txt, error_2.graphite.txt, rồi error_1-and-3.graphite.txt, và kéo một điểm trên hình elip màu đen bằng công cụ path. Hoàn tất khi nút không mong muốn không được tạo và tài liệu được render chính xác.

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