Refactor MPIPtr cconvert to support Julia 1.14

[nhz2]

Alternative to #935

CC @eschnett @giordano @vchuravy

[eschnett]

One of the main features of ccall and its automatic conversion to pointers is that it's GC safe, i.e. that it holds on to the respective Julia objects while a pointer to the object is in use. I didn't follow the logic that you're introducing, but since this kind of error (losing a GC root) is very difficult to diagnose, I want to recommend to add respective comments to the code, stating which part of the machinery is holding on to objects, which functions are called before objects are converted to pointers, and which are called on pointers.

[nhz2]

I've added some comments using Claude Code

[eschnett]

Thanks! The description sounds correct, and I like the ASCII art.