gh-150355: Allow creating tasks in cancelled TaskGroup with cooperative cancellation

[azibom]

gh-150355: Allow create_task after tg.cancel() – cooperative cancellation

Problem:
After tg.cancel(), create_task() raises RuntimeError: TaskGroup is shutting down. The test TODO says this is not ideal – the task should run until the first await and then be cancelled.

Solution:
Add _explicitly_cancelled flag. On explicit cancel() the flag is set. In create_task(), if _aborting and _explicitly_cancelled is true, the task is created normally and then scheduled for cancellation with call_soon. This allows the synchronous code up to the first await to run. Failure‑induced abort still raises RuntimeError.

Testing:
All 118 tests in test_asyncio.test_taskgroups pass.

[picnixz]

We first need a discussion before a PR. This is premature. Please read the devguide to understand our process.

This also needs:

• A doc change
• What's New.
• NEWS entries.

For now, I'm going to close it. Wait on the issue before re-opening the PR please.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.