gh-157044: Fix asyncio call graph truncation at aiter(callable, stop)

[KingLizard1020]

The two-argument form of aiter() awaits an
async_callable_iterator_awaitable that previously exposed nothing about
the callable's coroutine, so asyncio.print_call_graph() stopped at
worker().

This exposes the wrapped awaitable as a read-only aw_wrapped member
(same approach as gh-156980's ag_gen) and steps over it in
asyncio.graph._build_graph_for_future.

Fixes #157044

• Issue: asyncio.print_call_graph() loses the call stack at aiter(callable, stop_value) #157044