asyncio staggered race missing calls to asyncio.future_add_to_awaited_by() and asyncio.future_discard_from_awaited_by()

[graingert]

Bug report

Bug description:

asyncio staggered race missing calls to asyncio.future_add_to_awaited_by() and asyncio.future_discard_from_awaited_by()

A possible oversight in https://github.com/python/cpython/pull/124640/files

CPython versions tested on:

3.14

Operating systems tested on:

No response

Linked PRs

• gh-129195: use future_add_to_awaited_by/future_discard_from_awaited_by in asyncio.staggered.staggered_race #129253

[graingert]

Worth landing https://github.com/python/cpython/pull/128475/files first