Manual sweep of !74060 from 24.0 to main: Making RHadrons G4Exension more robust (ATLASSIM-7274)

John Derek Chapmanrequested to merge
jchapman/athena:Sweep74060from24.0tomain into main

In ATLMCPROD-11193 @mnovak identified an issue with how CustomParticles used by the RHadronsPhysicsTool are created leading to an inconsistency in the processes associated with them in Worker threads. This MR should address these issues.

The original MR !74060 (merged) missed being swept to main in !74373 (merged) due to non-trivial conflicts, hence the need for this manual sweep.

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