Core: Base of the Mirror object does not respect the visbility state of group it is inside of

### Problem description

`Mirror` object does not respect the visibility state of group it is inside of. It could be `Std_Group` and also building container in the BIM workbench, which makes very limited use of this tool for BIM modeling.

I am able to reproduce in FC 1.1.3

### Workbench affected?

Core

### Steps to reproduce

- open sample file (remove the .txt extension)

[FC_bim-container-mirror.FCStd.txt](https://github.com/user-attachments/files/31226306/FC_bim-container-mirror.FCStd.txt)

- hide the `Group` object

### Expected behavior

Both walls in the group (the base and the mirrored one) should be hidden.

### Actual behavior

Only the mirrored wall is hidden.

See behavior in the video:

https://github.com/user-attachments/assets/8e2ce077-c183-4df5-84d5-0e33ab53f059

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

```shell
OS: Debian GNU/Linux 13 (trixie) (GNOME/gnome-xorg/xcb)
Architecture: x86_64
Version: 26.3.0dev.20260812 (Git shallow) AppImage
Build date: 2026/08/12 05:41:29
Build type: Release
Branch: (HEAD detached at dc1ef6e88)
Hash: dc1ef6e880d6f5cf5d5a6eb3749373141eb0deb8
Python 3.11.14, Qt 6.8.3, Coin 4.0.10 (bundled), Pivy 0.6.11 (bundled), Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, Clipper2 , IfcOpenShell 0.8.0, OCC 7.8.1
Locale: English/Ireland (en_IE)
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Light/
QPA/File dialog/Color dialog: xcb/via Qt/via Qt
Navigation Style/Orbit Style/Rotation Mode: Revit/Free Turntable/Object center
Logical DPI/Physical DPI/Pixel Ratio: 96/158.196/1
OpenGL version/vendor/renderer: 4.6 (Compatibility Profile) Mesa 25.0.7-2+deb13u1/Intel/Mesa Intel(R) HD Graphics 4600 (HSW GT2)
```

### Last known good version (optional)

```shell

```

Start by reproducing the issue with the attached FC_bim-container-mirror.FCStd sample: hide the Group object and compare the base and mirrored walls. Trace the Core Mirror and group visibility handling; done means both walls become hidden when their containing group is hidden.

Written by the indexing model from the issue text.