[3.14] gh-119592: gh-152967: Fix ProcessPoolExecutor stranding submitted work when a max_tasks_per_child worker exits (GH-152978)

[hugovk]

gh-119592: Fix ProcessPoolExecutor stranding submitted work when a max_tasks_per_child worker exits

Worker replacement went through the executor object: the manager thread read executor attributes that shutdown(wait=False) clears concurrently, and could not replace workers at all once the executor was garbage collected. A worker exiting at its max_tasks_per_child limit in those states left the remaining submitted work permanently unexecuted and hung interpreter exit; the racing case could crash the manager thread.

Replace workers from the executor manager thread using its own state plus configuration read through the live executor weakref, which shutdown() never clears:

• After shutdown(wait=False) with the executor still referenced, a replacement is spawned and the remaining work is executed as documented.
• Once the executor has been garbage collected (ProcessPoolExecutor: pending futures never complete if executor is gc'd while a max_tasks_per_child= worker exits #152967), or a replacement worker cannot be started and no workers remain, the remaining futures now fail with BrokenProcessPool instead of never resolving.
• A new _force_shutting_down flag stops both spawn paths from starting workers that would escape terminate_workers()/kill_workers(). (cherry picked from commit 0c6422f)

Reviewed-multiple-times-by: Gregory P. Smith