Highlight-regexp does not work in REPL buffer

[clyfe]

Expected behavior

When I M-x: highlight-regexp a word in the REPL buffer, I expect it to show up highlighted. hi-lock-mode is enabled.

Actual behavior

The word is not highlighted.

Steps to reproduce the problem

1. Output something in the REPL buffer.
2. M-x highlight-regexp and type a word from the buffer.
3. The word is now still not highlighted.

Environment & Version information

CIDER version information

;; CIDER 1.20.0 (Lanzarote), nREPL 1.5.1
;; Clojure 1.12.0, Java 21.0.9

Lein / Clojure CLI version

Clojure CLI version 1.12.0.1479

Emacs version

30.2

Operating system

Ubuntu 24.04

JDK distribution

openjdk version "21.0.9" 2025-10-21

[bbatsov]

Related to #3787