Fix wrong field-free media assignment in the beam pipe

[sawenzel]

This fixes a problem in the beam pipe media assignment, diagnosed with the o2-sim-geometry-doctor tool (#15683). 22 volumes of the RB24 warm module, its ion pump and its B1 bellow, and the air shell at r = 79-80 cm, carried media labelled out-of-field, whereas their placements at z = 400-490 cm are inside the 4.2-4.6 kGauss solenoid fringe field. Geant4 honours the label and moved particles through these volumes on straight lines, about 171000 steps per min-bias event.

This is now fixed by assigning the in-field counterpart media at the construction site of these volumes. After the change no step in a field-free volume sees a non-zero field. The same volumes are also placed far down the beam line where the field really is zero. These placements now integrate the field too, which is correct but slower. A follow-up gives them cloned volumes of their own and restores the shortcut.

Related tickets:
https://its.cern.ch/jira/browse/O2-174
https://its.cern.ch/jira/browse/O2-175

[amorsch]

This was probably done intentionally, but I'm also not against fixing it. The field component transverse to the particle direction is much smaller than 0.5 T and has little effect on downstream detectors.

[sawenzel]

Thanks @amorsch : If there is a good reason for this (faster sim?) we can of course keep it. I could benchmark the difference it makes but maybe for consistency reasons it could be worth the simplification.

[amorsch]

I agree.
Probably more important for the pipe geometry used with FOCAL.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 478932f at 2026-08-17 22:02:

## sw/BUILD/O2Physics-latest/log
c++: fatal error: Killed signal terminated program cc1plus
ninja: build stopped: subcommand failed.

Full log here.

[sawenzel]

putting on hold (awaiting further benchmark studies + release validation)