datatype: Fix pack external by using the MPIR layer

[dalcinl]

Pull Request Description

Fixes #7313.

This PR could be an opportunity to refactor/consolidate the implementation and use of the following two routines.

// src/mpi/datatype/typerep/src/typerep_util.h
bool MPII_Typerep_basic_type_is_complex(MPI_Datatype el_type);
bool MPII_Typerep_basic_type_is_unsigned(MPI_Datatype el_type);

@hzhou Feel free to push/force-push to my branch, or pick the commit and continue in your clone.

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[dalcinl]

As the regression this PR is fixing was not catched by MPICH tests, I guess mpi4py is the best we have.
Test are passing in my workstation with ch3:nemesis, but let's run a more comprehensive CI matrix:
https://github.com/mpi4py/mpi4py-testing/actions/runs/13841348758

[hzhou]

LGTM. I'll run CI tests and merge after it clears.

@dalcinl Sorry for neglecting your issue. Thanks for the fix!

[hzhou]

This PR could be an opportunity to refactor/consolidate the implementation and use of the following two routines.

Good point! I'll review and make a new PR if we decide to do the clean up.

[hzhou]

test:mpich/warnings
test:mpich/ch3/most

[dalcinl]

@hzhou Looks like something is off in your Jenkins server.

[hzhou]

test:mpich/warnings

[hzhou]

@hzhou Looks like something is off in your Jenkins server.

They occasionally have hiccups 😄