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

issue で参照されている Flatten Path ノードの作成箇所を、utility_types.rs から確認します。クリーンな Graphite インスタンスで error_1-and-3.graphite.txt、error_2.graphite.txt、続けて error_1-and-3.graphite.txt を開き、パスツールで黒い楕円の点をドラッグして再現します。予期しないノードが作成されず、ドキュメントが正しくレンダリングされれば完了です。

索引モデルが issue の本文から書いたものです。