Tutorial reconstruction example does not work for me

### Checklist

- [x] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [x] For Python issues, I have tested with the [latest development wheel](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [x] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### Describe the issue

Built from source on Ubuntu 24.

```
* Version 0.19.0+fb693b6c9
```

So far everything works except for the reconstruction.

Following
https://www.open3d.org/docs/release/tutorial/reconstruction_system/system_overview.html

I am not sure if this tutorial refers to `t_reconstruction_system` or `reconstruction_system`. In either cases it did not work for me.

I ran
```
python run_system.py --make --register --refine --integrate
```

it bombs with
```
Traceback (most recent call last):
File "/home/sb/Open3D/examples/python/reconstruction_system/run_system.py", line 116, in
register_fragments.run(config)
File "/home/sb/Open3D/examples/python/reconstruction_system/register_fragments.py", line 203, in run
make_posegraph_for_scene(ply_file_names, config)
File "/home/sb/Open3D/examples/python/reconstruction_system/register_fragments.py", line 170, in make_posegraph_for_scene
initializer=limit_open3d_threads) as pool:
^^^^^^^^^^^^^^^^^^^^
NameError: name 'limit_open3d_threads' is not defined
```

I copied the file `examples/python/open3d_example.py` to `examples/python/reconstruction_system`.

It completes the run without an error, but does not work, meaning

1. It has 000 fragments
```
Fragment 000 / 029 :: integrate rgbd frame 99 (100 of 100).
====================================
Elapsed time (in h:m:s)
====================================
- Making fragments 0:01:31.343648
- Register fragments 0:00:00.012145
- Refine registration 0:00:00.007886
- Integrate frames 0:00:00.713844
- SLAC 0:00:00
- SLAC Integrate 0:00:00
- Total 0:01:32.077523
```

2. In debug mode the viewer opens an empty frame

No .ply file written.

### Steps to reproduce the bug

```python
See above
```

### Error message

See above

### Expected behavior

It should work according to the tutorial page.

### Open3D, Python and System information

```markdown
I don't think the versions are relevant, everything runs correctly up to this point.
```

### Additional information

I am not sure that copying or moving `open3d_example.py` is the real solution, I just wanted to see the reconstruction running correctly.

Start with the reconstruction tutorial and compare the referenced t_reconstruction_system and reconstruction_system examples. Trace run_system.py into register_fragments.py, then check how open3d_example.py is being used; reproduce the command and verify that the workflow completes without the NameError, creates fragments, and writes the expected .ply output.

Written by the indexing model from the issue text.