[BUG]: mcrun --mpi=auto does not function on Windows/conda with MSMPI

[willend]

Describe the bug
Automatic selection of number of MPI cores (--mpi=auto) fails on Windows with MSMPI.

Workaround: Use an actual number (4, 8,…)

Document steps to reproduce the bug
Steps to reproduce the behavior:
Run any instrument with MPI and ‘auto’ number of cors, you will get

INFO: ===
INFO: Using system default number of mpirun -np processes
ERROR: Error reported: failed to launch ' BOA_beamline_monitors.exe --trace=0 --ncount=1000000.0 --dir=C:\Users\peterwillendrup\AMG-Beamlines-McStas\BOA\BOA_beamline_monitors_20260311_123453 --format=McCode --bufsiz=10000000 flagin=0 trompete_exit_slit_width=0.04 trompete_exit_slit_height=0.04 Lmin=1 Lmax=9 threshold=1e-12' on WIN11ARM2025
Error (3) The system cannot find the path specified.
INFO: call to mpiexec failed with Command 'mpiexec "" BOA_beamline_monitors.exe --trace=0 --ncount=1000000.0 --dir=C:\Users\peterwillendrup\AMG-Beamlines-McStas\BOA\BOA_beamline_monitors_20260311_123453 --format=McCode --bufsiz=10000000 flagin=0 trompete_exit_slit_width=0.04 trompete_exit_slit_height=0.04 Lmin=1 Lmax=9 threshold=1e-12' returned non-zero exit status 4294967292.

Expected behavior
Let auto mean that Python probes the number of available cores, e.g. via os.cpu_count()

OS and McStas/McXtrace version (please complete the following information):

• OS: Windows - any version
• McCode variant: McStas or McXtrace alike
• Version: 3.x.y

Additional context
Issue spotted with Dylan C Bailey