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, 1 insertions, 1 deletions

diff --git a/tests/test-custom-comments-comment-block-banner.el b/tests/test-custom-comments-comment-block-banner.el
index 6561ebfa..da0b1fa7 100644
--- a/tests/test-custom-comments-comment-block-banner.el
+++ b/tests/test-custom-comments-comment-block-banner.el
@@ -194,7 +194,7 @@ Returns the buffer string for assertions."
   "Should error when decoration-char is nil."
   (should-error
    (test-block-banner-at-column 0 "/*" "*/" nil "Header" 70)
-   :type 'wrong-type-argument))
+   :type 'user-error))
 
 (ert-deftest test-block-banner-c-nil-text ()
   "Should error when text is nil."