TypeError is raised when a thread is shutdown via `Thread#kill`

[0x1eef]

Hey.

I came across this potential bug earlier today.

I'm not sure if this is an Ruby, async, or io-events problem.
It includes all three. It involves a separate thread, Async::Reactor,
and the $! variable. Under certain circumstances it appears that
$! can be the number 8 and it can propagate as the target cause
for a raise call during thread shutdown initiated via Thread#kill.

In other words it appears raise(ex, cause: 8) is being called on the
thread that is being shutdown.

repro.rb

##
# Reproduction
# Ruby 4.0.6, async 2.45.1, io-event 1.21.1.

require "async"
Async::Scheduler.prepend(Module.new do
  define_method(:raise) do |*args, **kw|
    warn "scheduler#raise: args=#{args.inspect} kw=#{kw.inspect}"
    super(*args, **kw)
  end
end)

th = Thread.new do
  reactor = Async::Reactor.new
  reactor.async do
    reactor.async { sleep 30 } 
    sleep 0.05
  end
  reactor.run
end
th.report_on_exception = false
sleep 0.2
th.kill
##
# re-raises on caller thread
th.join(1)

puts "thread alive? #{th.alive?}"

##
# scheduler#raise: args=[#<Fiber ...>, Async::Cancel] kw={cause: 8}
# TypeError: exception object expected (on the killed thread)