src: treat signal-terminated tasks as failures

[Archkon]

Check term_signal when handling task runner process exits so scripts terminated by a signal do not incorrectly report success.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (bb76938) to head (7c56ad3).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64951      +/-   ##
==========================================
+ Coverage   90.28%   90.29%   +0.01%     
==========================================
  Files         760      760              
  Lines      247137   247137              
  Branches    46617    46618       +1     
==========================================
+ Hits       223122   223152      +30     
+ Misses      15493    15459      -34     
- Partials     8522     8526       +4     

Files with missing lines	Coverage Δ
src/node_task_runner.cc	89.32% <100.00%> (ø)

... and 38 files with indirect coverage changes

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