gh-150058: asyncio: preserve Task.cancel() msg through fut_waiter and CancelledError paths

[prince8273]

When Task.cancel(msg=...) is called while the task is waiting on another
future, the message was silently discarded in two places:

1. cancel(): the fast path (_fut_waiter.cancel(msg) returns True)
   returned immediately without storing msg in _cancel_message.

2. __step_run_and_handle_result(): the CancelledError handler called
   future_cancel(..., NULL), dropping _cancel_message even when set.

Fix both in tasks.py (_PyTask) and _asynciomodule.c (Task).

Also includes:

• Regression test in Lib/test/test_asyncio/test_tasks.py:
  test_task_cancel_waiter_future_with_message covers the full
  round-trip — cancel with msg while awaiting a future, verify the
  message reaches the final CancelledError.
• What's New entry in Doc/whatsnew/3.16.rst.

• Issue: asyncio.Task.cancel() silently drops msg when cancelling via a waiter future #150058

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Documentation build overview

📚 cpython-previews | 🛠️ Build #32756060 | 📁 Comparing 8f63ba1 against main (517d3d2)

  🔍 Preview build  

4 files changed

± library/shutil.html
± library/zipfile.html
± whatsnew/3.16.html
± whatsnew/changelog.html

[picnixz]

Please stop updating your branch, it wastes CI resources. Read the devguide to understand our guidelines.

[prince8273]

Closing this PR. After testing on the CPython main branch locally, the cancel message is correctly preserved in CancelledError.args. The bug I reported does not reproduce — the fix is not needed. Apologies for the noise.