Avoid PassThrough to avoid buffering in pipeline

In the new async-terator capable pipeline as implemented in #31223, we support passing in an async generator function.
Reading from

Line 191 in 6bcea0a

const pt = new PassThrough();

, it seems we are always wrapping it in a stream internally. This adds overhead and another level of buffering.

We should avoid wrapping it in a PassThrough to avoid said buffering and overhead.