Fix anext awaitable ag gen

[anshuman0123]

Description

Fixes gh-157032.

When calling anext(agen, default) with a default value, Python wraps the underlying anext() awaitable in an internal
anext_awaitable instance (iterobject.c:406-410 in Objects/iterobject.c).

During task call-stack inspection, _build_graph_for_future in graph.py:40-82 traverses cr_await, ag_await, and ag_gen to walk
through coroutines, async generators, and generator-driving awaitables. Because anext_awaitable lacked an ag_gen attribute,
traversal terminated early upon encountering anext_awaitable, losing the entire call stack inside the async generator and any
child coroutines it was waiting on.

This PR exposes a read-only ag_gen attribute on anext_awaitable that delegates to self.wrapped.ag_gen.
──────

Changes Made:

1. iterobject.c:
   • Added getter anextawaitable_getag_gen to read ag_gen from obj->wrapped.
   • Added anextawaitable_getsetlist and hooked it into _PyAnextAwaitable_Type.tp_getset.
2. graph.py:
   • Updated the comment on ag_gen traversal to reference both asyncio.print_call_graph() loses the call stack at asynchronous generators #156980 and asyncio.print_call_graph() loses the call stack at anext(agen, default) #157032.
3. test_asyncgen.py:
   • Added test_async_gen_anext_default_ag_gen to verify anext(agen, default).ag_gen returns the generator, is read-only, and
   raises AttributeError for non-generator async iterators.
4. test_graph.py:
   • Added test_stack_async_gen_anext_default testing both C and Python event loop implementations.
5. functions.rst:
   • Documented the ag_gen attribute on the awaitable returned by anext().
6. Misc/NEWS.d/next/Library/:
   • Added NEWS entry for asyncio.print_call_graph() loses the call stack at anext(agen, default) #157032.

──────

Repro & Verification

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()
    try:
        await t
    except asyncio.CancelledError:
        pass

asyncio.run(main())

Before:

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

After:

* Task(name='worker', id=0x244549256d0)
  + Call stack:
  |   File '.../Lib/asyncio/locks.py', line 210, in async Event.wait()
  |   File 'repro.py', line 5, in async deep()
  |   File 'repro.py', line 8, in async generator rows()
  |   File 'repro.py', line 12, in async worker()

──────

Test Suite Results:

• python -m test test_asyncgen: Passed (100%)
• python -m test test_asyncio: Passed (35/35 test suites, 2,749 tests, 0 failures)

[picnixz]

The issue says:

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

Don't open issues when the OP already suggests to open a PR as it's rude, especially if such PRs are fully AI generated.