refactor(custom-editing): five hygiene fixes from the module-by-module re-review

- Guard `cj/duplicate-line-or-region' when COMMENT is non-nil but the
  current mode has no `comment-start' (e.g. fundamental-mode).
  Previously the function silently produced malformed output via
  `comment-region'; now it signals a clear `user-error'.

- Factor the `find-file' advice install in external-open.el into
  `cj/external-open-install-advice'.  Same idempotent shape
  (remove-then-add) but the intent is named.

- Add `cj/--validate-decoration-char' in custom-comments.el and
  wire it into all six divider / border / box helpers.  Rejects
  multi-char strings, empty strings, and control characters like
  newline/tab that would corrupt subsequent `M-q' flows.  Updated
  the five nil-decoration ERT tests from `:type 'wrong-type-argument'
  (the old crash signal from `string-to-char' on nil) to
  `:type 'user-error', since the validator produces a clear
  message instead of a deep crash.

- Extract `cj/--require-spell-checker' in flyspell-and-abbrev.el.
  Both `cj/flyspell-toggle' and `cj/flyspell-then-abbrev' now call
  the shared helper; the checker list lives in
  `cj/--spell-checker-executables', so adding nuspell or any other
  checker is a one-line edit.

- Preserve trailing newlines in custom-ordering output.  Both
  `cj/--arrayify' and `cj/--unarrayify' now detect a trailing
  newline on the input region and re-append it to the result,
  matching the pattern custom-text-enclose.el already uses.

1 files changed, 17 insertions, 12 deletions

diff --git a/modules/flyspell-and-abbrev.el b/modules/flyspell-and-abbrev.el
index e29ad6e9f..e732ac469 100644
--- a/modules/flyspell-and-abbrev.el
+++ b/modules/flyspell-and-abbrev.el
@@ -43,6 +43,8 @@
 
 ;;; Code:
 
+(require 'cl-lib)
+
 ;; Forward declarations
 (eval-when-compile (defvar org-dir))
 (defvar flyspell-mode)
@@ -111,18 +113,26 @@
 ;; ------------------------------ Flyspell Toggle ------------------------------
 ;; easy toggling flyspell and also leverage the 'for-buffer-type' functionality.
 
+(defconst cj/--spell-checker-executables
+  '("aspell" "ispell" "hunspell")
+  "Spell-checker executables `cj/--require-spell-checker' looks for.")
+
+(defun cj/--require-spell-checker ()
+  "Signal `user-error' unless a known spell-checker is on PATH.
+Looked-up executables: `cj/--spell-checker-executables'.  Single point
+of change when a new checker (e.g. nuspell) is added."
+  (unless (cl-some #'executable-find cj/--spell-checker-executables)
+    (user-error
+     "No spell checker found. Install one of: %s"
+     (mapconcat #'identity cj/--spell-checker-executables ", "))))
+
 (defun cj/flyspell-toggle ()
   "Turn Flyspell on if it is off, or off if it is on.
 
 When turning on, it uses `cj/flyspell-on-for-buffer-type' so code-vs-text is
 handled appropriately."
   (interactive)
-  ;; Check if spell checker is available
-  (unless (or (executable-find "aspell")
-              (executable-find "ispell")
-              (executable-find "hunspell"))
-    (user-error "No spell checker found. Install aspell, ispell, or hunspell"))
-
+  (cj/--require-spell-checker)
   (if (bound-and-true-p flyspell-mode)
       (progn ; flyspell is on, turn it off
         (flyspell-mode -1)
@@ -208,12 +218,7 @@ Without prefix argument, it's created in the global abbrev table.
 
 Press C-' repeatedly to step through misspellings one at a time."
   (interactive "P")
-  ;; Check if spell checker is available
-  (unless (or (executable-find "aspell")
-              (executable-find "ispell")
-              (executable-find "hunspell"))
-    (user-error "No spell checker found. Install aspell, ispell, or hunspell"))
-
+  (cj/--require-spell-checker)
   ;; Run flyspell-buffer only if buffer hasn't been checked yet
   (unless (bound-and-true-p flyspell-mode)
     (flyspell-buffer))