Use mpi-extensions main Open MPI in CI

[allnes]

What changed

• Installs Open MPI from the learning-process/mpi-extensions main prerelease in Linux/macOS CI instead of package-manager MPI.
• Adds scripts/install_mpi_extensions.py and a reusable setup action wrapper to download, verify, and unpack the selected release asset.
• Requires -D PPC_MPI_EXTENSIONS_HOME=/path/to/mpi-extensions-openmpi for Linux/macOS CMake builds when C++ components are enabled.
• Teaches CMake to select the MPI wrappers from that prefix and emit ppc_mpi_runtime_env.json; scripts/run_tests.py consumes that file and sets the required runtime environment itself.
• Leaves Windows MPI discovery unchanged.

User impact

A developer or CI job can unpack the mpi-extensions Open MPI archive anywhere and pass only:

cmake -S . -B build -D PPC_MPI_EXTENSIONS_HOME=/path/to/mpi-extensions-openmpi

No Homebrew/apt MPI install and no manual PATH, LD_LIBRARY_PATH, DYLD_LIBRARY_PATH, MPI_HOME, or OPAL_PREFIX exports are required for Linux/macOS course builds/tests.

Validation

• macOS arm64: installed the current mpi-extensions main Open MPI package, verified CMake fails without PPC_MPI_EXTENSIONS_HOME, configured/built/installed PPC_TASKS=example with ccache, and ran MPI tests via scripts/run_tests.py without MPI exports.
• Linux x86_64 (ubuntu-x86): installed the current mpi-extensions main Open MPI package, verified CMake fails without PPC_MPI_EXTENSIONS_HOME, configured/built/installed PPC_TASKS=example with ccache, and ran MPI tests via scripts/run_tests.py without MPI exports.
• Local static checks: isort, ruff, ruff format --check, flake8, cmake-format, yamllint, check-yaml, editorconfig-checker, py_compile, git diff --check, and actionlint for workflows.

Notes

pre-commit run as a single full command is blocked locally by this Mac's Node/npm engine mismatch in the markdownlint hook. The affected Python/CMake/YAML/editorconfig hooks were run directly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.63%. Comparing base (5f1e6ac) to head (e19dd76).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #834   +/-   ##
=======================================
  Coverage   90.63%   90.63%           
=======================================
  Files          15       15           
  Lines         491      491           
  Branches      183      183           
=======================================
  Hits          445      445           
  Misses          8        8           
  Partials       38       38           

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[allnes]

@aobolensk please review