bug#67180: 30.0.50; 'pp-to-string' emits extra newline

[Eshel Yaron via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On Emacs 29 and earlier, with `-Q`, we have:

(pp-to-string "foo")
  => "\"foo\""

On master with `-Q`, we get an extra newline at the end of the string:

(pp-to-string "foo")
  => "\"foo\"
"

AFAICT this change in behavior breaks `agda2-mode` completely on master.
This happens because apparently `agda2-mode` uses `pp-to-string` to
quote strings before sending them to a REPL (that doesn't expect the
newline the middle of a command):

--8<---------------cut here---------------start------------->8---
(defun agda2-string-quote (s)
  "..."
  (let ((pp-escape-newlines t)
        (s2 (copy-sequence s)))
    (set-text-properties 0 (length s2) nil s2)
    (mapconcat 'agda2-char-quote (pp-to-string s2) "")))
                                 ^^^^^^^^^^^^^^^^^
--8<---------------cut here---------------end--------------->8---


In GNU Emacs 30.0.50 (build 24, x86_64-apple-darwin23.0.0, NS
 appkit-2487.00 Version 14.0 (Build 23A344)) of 2023-11-13