PartDesign: SubShapeBinder support property has inconsistent behaviour when support is lost

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Problem description

There are at least three different behaviours for the support property when the support is lost:

1. Support becomes empty, if the support and binder are in the same body
2. Support is unchanged, if the support and binder are in different bodies
3. Sometimes the support becomes the parent object

In all these cases no errors are emitted.

The support property should have consistent behaviour when the support is lost, emit an error, and have an indicator on the tree icon so that missing supports can be more easily identified and fixed.

### Full version info

```shell
OS: Windows 10 build 19045
Architecture: x86_64
Version: 1.1.0dev.14555 (Git shallow)
Build date: 2025/10/07 22:49:55
Build type: Release
Branch: main
Hash: 68f083a6d169ec523fa9d46b7cc40addb1e4ffbe
Python 3.11.13, Qt 6.8.3, Coin 4.0.3, Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, IfcOpenShell 0.8.2, OCC 7.8.1
Locale: English/United States (en_US)
Navigation Style/Orbit Style/Rotation Mode: Gesture/Trackball/Object center
Stylesheet/Theme/QtStyle: FreeCAD Light.qss/FreeCAD Light/
Logical DPI/Physical DPI/Pixel Ratio: 96/108.888/1.5
```

### Subproject(s) affected?

PartDesign

### Anything else?

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct