investigate mpi-parts mpi-parts_xios ratio

#470 && #519 introduced a seeming constraint for large weekly jobs that

* the number of `mpi_parts` now needs to be exactly divisible by the number of `mpi_parts_xios`
* it is not at all clear what has re-introduced this constraint, which was suggested in the code base a long time ago, but has not been observed on `main` at all recently.
* it is not clear whether this constraint is also linked to scale

investigate this constraint, and whether it applies to all jobs, or at some scale

implement rose-stem or launch-exe level testing that will fail for jobs that do not meet this constraint and would fail, with clear error messaging

Start by reading issues #470 and #519 and tracing where mpi_parts and mpi_parts_xios are validated or used in rose-stem and launch-exe for large weekly jobs. Reproduce jobs with divisible and non-divisible ratios at different scales, then add a test that captures the observed failure cases and reports a clear constraint error.

Written by the indexing model from the issue text.