OCC: Inner thickness doesnt work as expected

### Problem description

A plane appears instead of desired part with the inner thickness. Outer thickness works fine.

It works fine for the outer thickness though.

I am using:

> OS: Windows 11 build 26200
> Architecture: x86_64
> Version: 1.1.1.20260414 (Git shallow)
> Build date: 2026/04/14 22:09:59
> Build type: Release
> Branch: (HEAD detached at 0108fd4b4)
> Hash: 0108fd4b4850cc46e625b60e53cea7a7bbe69f8d
> Python 3.11.15, 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 0.8.4, OCC 7.8.1
> Locale: Czech/Czechia (cs_CZ)
> Navigation Style/Orbit Style/Rotation Mode: Blender/Trackball/Drag at cursor
> Stylesheet/Theme/QtStyle: OpenLight.qss/OpenLight/
> Logical DPI/Physical DPI/Pixel Ratio: 96/108.888/1
> Installed mods:
> * Help 1.0.3
> * OpenTheme 2025.5.20

Including file with incorrect geometry:

[krabicka.zip](https://github.com/user-attachments/files/30164311/krabicka.zip)

### Workbench affected?

Part Design

### Steps to reproduce

Just click on Thickness in the Stack menu

### Expected behavior

Correct inner thickness geometry

### Actual behavior

It creates some plane above the model instead of walled geometry.

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

```shell
Safe mode is the same

OS: Windows 11 build 26200
Architecture: x86_64
Version: 1.1.1.20260414 (Git shallow)
Build date: 2026/04/14 22:09:59
Build type: Release
Branch: (HEAD detached at 0108fd4b4)
Hash: 0108fd4b4850cc46e625b60e53cea7a7bbe69f8d
Python 3.11.15, 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 0.8.4, OCC 7.8.1
Locale: Czech/Czechia (cs_CZ)
Navigation Style/Orbit Style/Rotation Mode: CAD/Rounded Arcball/Window center
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Light/
Logical DPI/Physical DPI/Pixel Ratio: 96/108.888/1
```

### Last known good version (optional)

```shell

```

Open the attached krabicka.zip model in the Part Design workbench and reproduce the issue by selecting Thickness from the Stack menu. Compare the incorrect plane produced by inner thickness with the working outer-thickness result; done means the inner option creates the expected walled geometry on the reported build.

Written by the indexing model from the issue text.