gh-146194: Fix nested KeyboardInterrupt handling in asyncio

- Modify _on_sigint to cancel main task on every SIGINT
- Allow nested cancellations to propagate correctly through multiple levels
- Add test_nested_keyboardinterrupt_handling to test_runners.py
- Add NEWS entry

Fixes issue where third Ctrl+C would crash with:
'Task was destroyed but it is pending!'

Author: okiemute04

Misc/NEWS.d/next/Library/2026-03-20-04-20-47.gh-issue-146194.aXjDnd.rst

@@ -2,10 +2,10 @@
 
 .. section: Library
 
-Fix nested :exc:`KeyboardInterrupt` handling in :mod:`asyncio`. 
+Fix nested :exc:`KeyboardInterrupt` handling in :mod:`asyncio`.
 Previously,
-multiple Ctrl+C presses would cause a crash with ``Task was 
+multiple Ctrl+C presses would cause a crash with ``Task was
 destroyed but it
-is pending!``. Now nested cancellations propagate correctly through 
+is pending!``. Now nested cancellations propagate correctly through
 multiple
 levels.