Assembly: link sub-elements on multi-element links which are inside an assembly can not be hidden (eyeball does not respond to clicks)

### Problem description

When using multi-element links inside an assembly you are not able to click the eyeball in the tree view next to that element to hide it. You can hide the link container, but not any of it's sub-elements.

This used to be possible because I have older documents containing hidden link elements and in those cases I'm unable to unhide them. It seems that something regressed which caused this specific combination to not respond.

### Workbench affected?

Assembly

### Steps to reproduce

1. Open the attached file:
[link_multi_instance_hide_bug.FCStd.zip](https://github.com/user-attachments/files/31804424/link_multi_instance_hide_bug.FCStd.zip)
2. Try to hide `Link_i0` in the tree

### Expected behavior

`Link_i0` is hidden exactly like `Link001_i0`

### Actual behavior

Clicking the eyeball on `Link_i0` does nothing

### Development version About Info (in Safe Mode)

```shell
OS: Arch Linux (sway/sway-vulkan/wayland)
Architecture: x86_64
Version: 26.3.0dev.48347 (Git)
Build date: 2026/08/30 21:26:30
Build type: Release
Branch: makepkg
Hash: abd82a25c9d60c25a341edf399710ae468182922
Python 3.14.7, Qt 6.11.2, Coin 4.0.10 (bundled), Pivy 0.6.11 (bundled), Vtk 9.6.2, boost 1_92, Eigen3 5.0.1, PySide 6.11.2
shiboken 6.11.2, SMESH 7.7.1.0, xerces-c 3.3.0, Clipper2 , OCC 7.9.3
Locale: English/United States (en_US)
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Dark/
QPA/File dialog/Color dialog: wayland/via Qt/via Qt
Navigation Style/Orbit Style/Rotation Mode: CAD/Rounded Arcball/Window center
Logical DPI/Physical DPI/Pixel Ratio: 96/129.001/1
OpenGL version/vendor/renderer: 4.6 (Compatibility Profile) Mesa 26.2.1-arch1.1/AMD/AMD Radeon Pro W6800 (radeonsi, navi21, ACO, DRM 3.64, 7.2.2-arch1-1)
```

### Last known good version (optional)

```shell
Due to another bug with Appimages on my current OS bisecting requires building everything from source. Unfortunately this is extremely time consuming so I can't provide any data here right now. Sorry.
```

Start with the attached link_multi_instance_hide_bug.FCStd file and reproduce the issue in the Assembly workbench's tree view, comparing Link_i0 with Link001_i0. Trace the tree-view handling for hiding multi-element link sub-elements and verify that clicking Link_i0's eyeball toggles visibility like Link001_i0.

Written by the indexing model from the issue text.