[BUG]: VTX write list of DG0 functions + other spaces does not work

[ordinary-slim]

Summarize the issue

The VTX writer of dolfinx writes a list of DG0 + other functions to a .bp folder without errors but ParaView errors when it tries to read it.

ParaView believes the DG0 functions are point data when they should be cell data.

The feature was broken by afec7b1 (2026-01-05).
It didn't break the test test_dg_0_data, as the test only writes the data but does
not verify it can be read.

TODO:

• Fix bug
• Add reading to the test.

I will make a PR later today if it hasn't been addressed.

How to reproduce the bug

Add these two lines to dolfinx/python/test/unit/io/test_adios2.py:

if __name__=="__main__":
    TestVTX().test_dg_0_data(tempdir="out")

Run the file and attempt to read the bp folder in out with ParaView.

Minimal Example (Python)

Output (Python)

ParaView error:

ERROR: In vtkDataSet.cxx, line 708
vtkUnstructuredGrid (0x3615bb60): Point array q with 3 components, only has 50 tuples but there are 231 points

ERROR: In vtkPVGeometryFilter.cxx, line 877
vtkPVGeometryFilter (0x3911fa20): Input is not a vtkDataObjectTree.

Version

main branch

DOLFINx git commit

No response

Installation

Install from source on Linux machine.

Additional information

No response