Use `_PyInterpreterFrame.stackpointer` in free threaded GC when traversing threads' stacks

[colesbury]

Feature or enhancement

The free threading GC traverses the active frames of threads in two places:

• When GC_MARK_ALIVE_STACKS during the marking phase (see Add marking phase to free-threaded cyclic GC #128807)
• During deduce_unreachable_heap to ensure that deferred reference counted objects are kept alive.

Previously, the frame's stackpointer was frequently not set during GC. To avoid missing some deferred references, we looked at the frame up to the maximum size (co->co_stacksize). That behavior isn't great:

• We can only look at deferred references, because non-deferred references may be dead objects reclaimed by normal reference counting operations.
• We need to zero-initialize the entire stack when pushing a frame

Now that the frame's stackpointer is nearly always set during GC, we should only look at the frame up to f->stackpointer. There are still some cases where the stackpointer isn't set currently; we'll (temporarily) need some fallback for those cases. Once the stackpointer is always valid during GC (ideally before the 3.14 release), we can remove the fallback and assert that f->stackpointer is not NULL.

cc @nascheme

Linked PRs

• gh-129236: Use stackpointer in free threaded GC #129240