Fix compatibility issues with fiber profiler.

[ioquatix]

For some reason, this test interacts badly with the fiber profiler:

{"start_time":0.000,"duration":0.011738,"calls":[],"switches":2,"samples":1,"stalls":1}
bundler: failed to load command: sus (/home/runner/work/async/async/vendor/bundle/ruby/3.4.0/bin/sus)
/home/runner/work/async/async/lib/async/scheduler.rb:369:in 'IO::Event::Selector::URing#io_write': Async::Stop (Async::Stop)
	from /home/runner/work/async/async/lib/async/scheduler.rb:369:in 'Async::Scheduler#io_write'
	from /home/runner/work/async/async/vendor/bundle/ruby/3.4.0/bin/sus:in 'IO#write'

https://github.com/socketry/async/actions/runs/16248374683/job/45874529801

For now, let's change it, and I can debug it later when I have more time.

Types of Changes

• Maintenance.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.