cython tasks are KI protected

if you write a python function:

# tests/cython/pyfunc.py

import threading

def sleep() -> None:
    threading.Event().wait()

and call it from a cython task:

# cython: language_level=3
import trio

from .pyfunc import sleep

# the output of the prints are not currently checked, we only check
# if the program can be compiled and doesn't crash when run.

# The content of the program can easily be extended if there's other behaviour
# that might be likely to be problematic for cython.
async def foo() -> None:
    print('.')

async def bad():
    sleep()

async def trio_main() -> None:
    print('hello...')
    await trio.sleep(1)
    print(' world !')

    async with trio.open_nursery() as nursery:
        nursery.start_soon(foo)
        nursery.start_soon(foo)
        nursery.start_soon(foo)

    await bad()

trio.run(trio_main)

it hangs forever and can't be interrupted with ctrl+c

currently_ki_protected sees the following stack:

frame=<frame at 0x71d9198c7920, file '/usr/lib/python3.12/threading.py', line 355, code wait>
frame=<frame at 0x71d9198edd80, file '/usr/lib/python3.12/threading.py', line 655, code wait>
frame=<frame at 0x71d91992ed40, file '/home/graingert/projects/trio/tests/cython/pyfunc.py', line 4, code sleep>
frame=<frame at 0x71d91a083040, file '/home/graingert/projects/trio/src/trio/_core/_run.py', line 2720, code unrolled_run> (protected=True)

Start with the Cython reproducer in tests/cython/pyfunc.py and trace the protected frame handling in src/trio/_core/_run.py, especially unrolled_run. Reproduce the hang and inspect how the threading wait is classified by currently_ki_protected. Done means the Cython task can be interrupted without hanging, with regression coverage for the reported behavior.