TaskInfo.has_pending_cancellation returns false positives in shielded cleanup on asyncio >= 3.11

[gschaffner]

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

AnyIO version

4.4.0, 4.6.2, master (0f80611)

Python version

3.11.10, 3.12.7, 3.13.0

What happened?

TaskInfo.has_pending_cancellation returns false positives in shielded cleanup (finally, etc.) on asyncio >= 3.11. I noticed this case while reading the implementation (haven't hit it IRL yet). Investigation indicates that:

• It was introduced (on all Python versions) with has_pending_cancellation in Fixed memory object stream sometimes dropping sent items #735.

• It was fixed on Python < 3.11 by Fix TaskInfo.has_pending_cancellation() false positives on asyncio with shielding #772.

• It is still present on Python >= 3.11.

How can we reproduce the bug?

async def test_shielded_cleanup() -> None:
    with CancelScope() as outer_scope:
        outer_scope.cancel()
        try:
            await checkpoint()
        finally:
            assert current_effective_deadline() == -math.inf
            assert get_current_task().has_pending_cancellation()

            with CancelScope(shield=True):  # noqa: ASYNC100
                assert current_effective_deadline() == math.inf
                assert not get_current_task().has_pending_cancellation()  # this one fails

            assert current_effective_deadline() == -math.inf
            assert get_current_task().has_pending_cancellation()

[gschaffner]

Closed by #790