Custom async-to-thread implementation, with signature async def (default_aio_to_thread, ctx, func, /, *args, **kwargs). default_aio_to_thread is LangSmith's default implementation, which the override can call to fall back to default behavior (e.g., when outside a constrained runtime context). ctx is the contextvars.Context LangSmith wants func to run inside; tracing state will be read back from this Context after the call. Override for runtimes like Temporal that don't support asyncio.run_in_executor.