Harden external-QM execution

[galjos]

External-QM runs now reject failed processes and incomplete result files. Executable and working-directory paths are quoted, including paths with spaces.

Validated by testExternalQMRunner and testStringUtilities.

Closes #381.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.13%. Comparing base (6148afd) to head (83a474e).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #384      +/-   ##
==========================================
+ Coverage   90.07%   90.13%   +0.05%     
==========================================
  Files         291      291              
  Lines       11845    11855      +10     
  Branches     3755     3761       +6     
==========================================
+ Hits        10669    10685      +16     
+ Misses       1132     1126       -6     
  Partials       44       44              

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[pq-perf-bot]

⚡ Performance (instruction count) — ✅ no regressions

per-benchmark breakdown

benchmark	base Ir	PR Ir	Δ
bondedForces	41.47M	41.47M	+0.00%	✅
boxTransforms	10.67M	10.67M	+0.00%	✅
constraints	11.41M	11.41M	-0.00%	✅
coulombKernel	5.80M	5.80M	+0.00%	✅
forceKernel	15.35M	15.35M	-0.00%	✅
integrator	32.19M	32.19M	+0.00%	✅
kinetics	9.53M	9.53M	+0.00%	✅
linearAlgebra	2.07M	2.07M	+0.00%	✅
nonCoulombPairs	5.48M	5.48M	+0.00%	✅
shiftVector	5.81M	5.81M	+0.00%	✅
virial	11.64M	11.64M	+0.07%	✅

_{Deterministic callgrind instruction counts vs the base branch; gated at ±2%. Not wall-clock.}

[ape33]

In principal a nice addition, but I have been working on the external QM runners in the qmmm-dev branch and dread the inevitable merge conflicts that will arise. Could we re-do this after merging the qmmm-dev into dev. I guess it is easier and less error-prone for the AI to re-do the changes than me resolving merge conflicts...

Approval dismissed: new commits contain changes beyond a merge-up from dev.