race condition when software-pwms stop and lgpio needs to clean up

If Python exits quickly, and lgpio hasn't cleaned, an error is thrown:

Exception in thread Thread-5 (by_duration):
Traceback (most recent call last):
  File "/usr/lib/python3.11/threading.py", line 1038, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.11/threading.py", line 975, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.11/dist-packages/pioreactor/actions/pump.py", line 106, in by_duration
    self.stop()
  File "/usr/local/lib/python3.11/dist-packages/pioreactor/actions/pump.py", line 85, in stop
    self.pwm.stop()
  File "/usr/local/lib/python3.11/dist-packages/pioreactor/utils/pwm.py", line 246, in stop
    self._pwm.off()
  File "/usr/local/lib/python3.11/dist-packages/pioreactor/utils/pwm.py", line 87, in off
    self.dc = 0.0
    ^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/pioreactor/utils/pwm.py", line 98, in dc
    lgpio.tx_pwm(self._handle, self.pin, self.frequency, self.dc)
  File "/usr/lib/python3/dist-packages/lgpio.py", line 1083, in tx_pwm
    return _u2i(_lgpio._tx_pwm(
           ^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/lgpio.py", line 458, in _u2i
    raise error(error_text(v))
lgpio.error: 'unknown handle'

Reproducible with: pio run add_alt_media --duration 1

The error only really happens when lgpio is inside a thread (like for pumps by_duration function).

Start with pioreactor/actions/pump.py, especially the by_duration and stop paths, then inspect pioreactor/utils/pwm.py where lgpio cleanup and PWM shutdown occur. Reproduce with pio run add_alt_media --duration 1 and trace the threaded shutdown. Done means quick Python exit no longer produces an lgpio.error: 'unknown handle' exception.