Fix MPI environment variable list size

[allnes]

Summary

• fix MPI environment variable list size in util.cpp
• document the array and ensure no empty entries

Testing

• pytest -q (no tests found)

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https://chatgpt.com/codex/tasks/task_e_68555dc14d648333b32dad97a979606a

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.41%. Comparing base (f6a8c83) to head (290c4b6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #455   +/-   ##
=======================================
  Coverage   92.41%   92.41%           
=======================================
  Files          11       11           
  Lines         356      356           
  Branches      125      125           
=======================================
  Hits          329      329           
  Misses         10       10           
  Partials       17       17           

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