`asyncio.print_call_graph()` loses the call stack at anext(agen, default)

[deadlovelll]

Bug report

Bug description:

Adding a default to anext() hides everything the generator is waiting on

Repro:

import asyncio

async def deep():
    await asyncio.Event().wait()

async def rows():
    await deep()
    yield 1

async def worker():
    await anext(rows(), None)

async def main():
    t = asyncio.create_task(worker(), name='worker')
    await asyncio.sleep(0.05)
    asyncio.print_call_graph(t)
    t.cancel()

asyncio.run(main())

Expected:

* Task(name='worker', id=0x103a049b0)
  + Call stack:
  |   File '/Users/timofeiivankov/cpython/Lib/asyncio/locks.py', line 210, in async Event.wait()
  |   File '/Users/timofeiivankov/cpython/repro.py', line 4, in async deep()
  |   File '/Users/timofeiivankov/cpython/repro.py', line 7, in async generator rows()
  |   File '/Users/timofeiivankov/cpython/repro.py', line 11, in async worker()

Actual:

* Task(name='worker', id=0x1035c89b0)
  + Call stack:
  |   File '/Users/timofeiivankov/cpython/repro.py', line 11, in async worker()

Proposed fix: expose ag_gen on anext_awaitable like #156984 do

Important: Currently this issue have a blocker from #156984 (issue - #156980) because it depends on the fix of it. I have a fix ready and will open a PR once #156984 is merged

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS