StreamReader::poll_next dropped the reader it take()s whenever the read
returned Poll::Pending, so the next poll reported end-of-stream. And
poll_read_buf measured bytes via ReadBuf::filled(), which never advances
when the reader writes through initialize_unfilled(), so every read
counted as zero bytes. Together these made any AsyncBody::from_reader
request body (e.g. one backed by async_fs::File, which returns Pending
on its first read) upload as an empty or truncated body.

Keep the reader across Pending polls and use the byte count that
futures::AsyncRead::poll_read returns.



- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

Co-authored-by: Marshall <marshall@zed.dev>