Starting from "emacs -Q":

   M-x eshell-command RET *echo hi | *cat & RET

You'll see the following error in the *Messages* buffer instead of the 
result:

   eshell-eval-command: Unmatched delimiter: (#<process echo> . 
#<process cat>)

This is a regression from my fix for bug#53715, which changed how
Eshell pipelines return the processes in the pipeline. Attached are some 
patches to fix it. One for the emacs-29 branch, and one for master.

Eli, is it ok to merge the patch for emacs-29? I tried to keep the 
change as minimal as possible for that branch.

The patch for master is a bit more extensive, and also fixes another 
issue where this would fail due to incorrect syntax in the Eshell 
command form:

   (eshell-command "*echo hi &" t)

Previously, it turned the command into "*echo hi & >>> 
#<current-buffer>", but that's not right; the "&" needs to go last.

For master, I also thought it would be nice to clean up 'eshell-command' 
slightly (see patch 0002); this just changes it to handle its arguments 
in the interactive spec.