BUG: Ensure remaining application events are processed before exiting

[jcfr]

This update ensures that any pending messages logged from Python scripts are properly reported and written to log files before the application exits.

This issue was uncovered while working on #7299

To illustrate:

Before

• ✅ With explicit call to slicer.app.processEvents():

  rm /tmp/Slicer-jcfr/*.log && \
  ./Slicer --python-code "import logging; logging.debug('EXPECTED DEBUG OUTPUT'); slicer.app.processEvents()" \
    --exit-after-startup 2>&1 > /dev/null && \
    (echo "Slicer exited without error" && cat $(ls -1 /tmp/Slicer-jcfr/*.log) | grep "EXPECTED DEBUG OUTPUT")
  
  [DEBUG][Python] 23.10.2023 17:35:16 [Python] (<string>:1) - EXPECTED DEBUG OUTPUT
  

• ❌ Without explicit call to slicer.app.processEvents():

  rm /tmp/Slicer-jcfr/*.log && \
  ./Slicer --python-code "import logging; logging.debug('EXPECTED DEBUG OUTPUT');" \
    --exit-after-startup 2>&1 > /dev/null && \
    (echo "Slicer exited without error" && cat $(ls -1 /tmp/Slicer-jcfr/*.log) | grep "EXPECTED DEBUG OUTPUT")
  

Message with EXPECTED DEBUG OUTPUT is missing from the log file

After

• ✅ With explicit call to slicer.app.processEvents():

  rm /tmp/Slicer-jcfr/*.log && \
  ./Slicer --python-code "import logging; logging.debug('EXPECTED DEBUG OUTPUT'); slicer.app.processEvents()" \
    --exit-after-startup 2>&1 > /dev/null && \
    (echo "Slicer exited without error" && cat $(ls -1 /tmp/Slicer-jcfr/*.log) | grep "EXPECTED DEBUG OUTPUT")
  
  [DEBUG][Python] 23.10.2023 17:39:25 [Python] (<string>:1) - EXPECTED DEBUG OUTPUT
  

• ✅ Without explicit call to slicer.app.processEvents():

  rm /tmp/Slicer-jcfr/*.log && \
  ./Slicer --python-code "import logging; logging.debug('EXPECTED DEBUG OUTPUT');" \
    --exit-after-startup 2>&1 > /dev/null && \
    (echo "Slicer exited without error" && cat $(ls -1 /tmp/Slicer-jcfr/*.log) | grep "EXPECTED DEBUG OUTPUT")
  
  [DEBUG][Python] 23.10.2023 17:38:59 [Python] (<string>:1) - EXPECTED DEBUG OUTPUT