Edgecentered 2d navigation agent path postprocessing does not give shortest path

### Tested versions

Godot custom built from master branch. (b432e108d50fd436d04d0f79b0a7cab8985032f7)

### System information

Windows 11

### Issue description

Seen on latest. On diagonal navigation, tends to snake around instead of giving shortest paths. Example screenshot taken from example project:

### Steps to reproduce

Have a isometric tilemap, and use a navigation agent on it configured to edge_centered postprocess. Navigate straight up/down or straight left/right (what would be diagonal on a traditional square tilemap)

### Minimal reproduction project (MRP)

[Project.zip](https://github.com/user-attachments/files/22140635/Project.zip)

Start with the attached Project.zip and reproduce the issue using an isometric tilemap and a navigation agent configured for edge_centered postprocessing. Compare the generated path when navigating vertically or horizontally, then trace the edge_centered navigation postprocessing entry point in Godot's navigation code. Done means those routes no longer snake and produce the shortest path.

Written by the indexing model from the issue text.