Test failure in test_pool_handles_transaction_exit_in_asyncgen_2 with Python 3.15.0a5, PYTHONASYNCIODEBUG=1

まだ誰も着手していません。

```
$ git clone https://github.com/MagicStack/asyncpg.git
$ cd asyncpg
$ git submodule update --init --recursive
$ uv venv --python 3.15
Using CPython 3.15.0a5 interpreter at: /usr/bin/python3.15
Creating virtual environment at: .venv
Activate with: source .venv/bin/activate
$ uv pip install -e .[gssauth]
$ uv pip install --group test
$ PYTHONASYNCIODEBUG=1 pytest -v
[…]
________________________________________________________________ TestPool.test_pool_handles_transaction_exit_in_asyncgen_2 _________________________________________________________________
Traceback (most recent call last):
File "/usr/lib64/python3.15/unittest/case.py", line 58, in testPartExecutor
yield
File "/usr/lib64/python3.15/unittest/case.py", line 667, in run
self._callTestMethod(testMethod)
~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
File "/usr/lib64/python3.15/unittest/case.py", line 613, in _callTestMethod
result = method()
File "/home/ben/src/forks/asyncpg/asyncpg/_testbase/__init__.py", line 92, in wrapper
self.loop.run_until_complete(coro)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
File "/usr/lib64/python3.15/asyncio/base_events.py", line 719, in run_until_complete
return future.result()
~~~~~~~~~~~~~^^
RuntimeError: cannot reuse already awaited coroutine
```

The above instructions also result in `TestConnectionAttributes.test_prefer_standby_picks_master_when_standby_is_down` failing due to a timeout, but I’m not seeing that when I build https://src.fedoraproject.org/rpms/python-asyncpg.

The test `TestPool.test_pool_handles_transaction_exit_in_asyncgen_2` passes when `PYTHONASYNCIODEBUG=1` is not set.

Python 3.15.0a5、PYTHONASYNCIODEBUG=1、および文書化されたpytestコマンドで失敗を再現します。まず TestPool.test_pool_handles_transaction_exit_in_asyncgen_2 と asyncpg/_testbase/__init__.py:92 から始め、次に asyncio のデバッグなしで実行した場合と比較し、別個の standby timeout を確認します。報告されたテスト失敗を理解して解決するか、明確に切り分けられれば完了です。

索引モデルが issue の本文から書いたものです。