GraphEdit: Improper minimap scaling when zooming

### Tested versions

Reproducible in Godot 4.6.1-stable & Godot 4.6.2-stable

### System information

Windows 10 - Godot v4.6.2-stable - Compatibility Renderer

### Issue description

When zooming in/out near the edge of the graph, the minimap applies incorrect scaling which results in the camera's rect to appear out of the bounding box of the minimap.

### Steps to reproduce

1. Instantiate a GraphEdit node
2. Pan camera to edge of GraphEdit
3. Zoom in, the camera rect appears to move outside of the minimap rect.

### Minimal reproduction project (MRP)

[GraphEdit MRP.zip](https://github.com/user-attachments/files/27623667/GraphEdit.MRP.zip)

Start with the attached GraphEdit MRP and reproduce the issue by panning to the edge, then zooming in and out. Trace the GraphEdit minimap and camera-rect behavior; done means the camera rect remains within the minimap bounds during edge zooming and the regression is covered by an appropriate test.

Written by the indexing model from the issue text.