BaseSolver.solve() multiprocessing is broken on system's defaulting to spawn context for windows

[kratman]

Follow up to #3975

It's worth noting that this only fixes https://github.com/pybamm-team/PyBaMM/issues/3974 for darwin systems and protects linux systems from the default context change in Python 3.14. Windows systems will still require the main guard discussed in https://github.com/pybamm-team/PyBaMM/issues/3974.

[agriyakhetarpal]

Labelled the issue with high priority so that we remember to get this done before v24.5rc0

[kratman]

@BradyPlanden Any update on this ticket?

[brosaplanella]

Following up on the dev meeting, we agreed to include a more informative error for Windows, and fixes can be done as part of #4087

[agriyakhetarpal]

Removed this from blocking the release based on comments from the dev meeting and on the issue linked above