Fix: when_all cancelled from environment - #1718

* Fix: when_all cancelled from environment

In the case when when_all is being stopped from a stop source in the
environment, the stop_source was triggered multiple times leading to a
use after free bug.

This patch fixes this by performing the following steps:

 - Inside the stop callback, increment the reference count.
 - Try setting the state to stopped, only if stopped, trigger the
   internal stop_source which cancels the children.
 - Arrive to decrement the count again and handle any concurrent
   completions if needed.

* use atomics from `stdexec::__std::` instead of `std::`

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Co-authored-by: Eric Niebler <[email protected]>

The stop callbacks that when_all (since NVIDIA#1718) and exec::when_any (since
NVIDIA#2115) register on the receiver's stop token increment the count of
outstanding children, request stop, and then arrive. If the last child
arrives on another thread while such a callback is running, the count is
already zero, so the callback's arrival completes the operation a second
time. With inplace_stop_source the thread of the last child usually
spins forever in ~inplace_stop_callback instead, waiting for a callback
that the other thread has already destroyed.

The extra count only worked around inplace_stop_source not tolerating
its destruction from inside its own request_stop() (NVIDIA#1889), which NVIDIA#2303
fixed. Make both callbacks only forward the request to the internal stop
source, like on-stop-request in [exec.when.all]. For when_all this also
undoes the rest of the workaround from NVIDIA#1851: it completes with
set_stopped only when a child does, and it no longer records values once
a child has failed or been stopped.

Co-Authored-By: Claude Opus 5.5 <[email protected]>