src: fix --process-timeout message when exit waits

[trivikr]

Fixes: #66360

process.exit(), uncaught exceptions and unhandled rejections exit through Environment::Exit(), which never calls OnEnvironmentStopping(). If a Worker blocked in a synchronous native call held up that exit past the deadline, the watchdog blamed the main thread.

Check Environment::is_stopping() and report that the process did not finish exiting instead.

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Assisted-by: claude:opus-5.5

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.36%. Comparing base (75e4bbe) to head (3ca961c).

Files with missing lines	Patch %	Lines
src/node_watchdog.cc	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66361      +/-   ##
==========================================
- Coverage   90.36%   90.36%   -0.01%     
==========================================
  Files         792      792              
  Lines      275398   275401       +3     
  Branches    52776    52773       -3     
==========================================
  Hits       248877   248877              
- Misses      16937    16939       +2     
- Partials     9584     9585       +1     

Files with missing lines	Coverage Δ
src/node_watchdog.cc	79.31% <0.00%> (-0.52%)	⬇️

... and 24 files with indirect coverage changes

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