The AsyncOpenAI client was created once at __init__ and stored as an
instance attribute. process_directory() calls asyncio.run() which creates
and closes a fresh event loop. On a second call, the client's httpx
transport is still bound to the closed loop, raising RuntimeError:
"Event loop is closed" — the same pattern fixed by PR #3398 for the
main agent loop.
Create the client lazily in _get_async_client() so each asyncio.run()
gets a client bound to the current loop.
Co-authored-by: binhnt92 <binhnt.ht.92@gmail.com>
