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.

6 files changed, 9 insertions, 7 deletions

diff --git a/tests/test-custom-comments-comment-block-banner.el b/tests/test-custom-comments-comment-block-banner.el
index 6561ebfab..da0b1fa7f 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."
diff --git a/tests/test-custom-comments-comment-box.el b/tests/test-custom-comments-comment-box.el
index 10b1a67d0..d7ee2830c 100644
--- a/tests/test-custom-comments-comment-box.el
+++ b/tests/test-custom-comments-comment-box.el
@@ -203,7 +203,7 @@ Returns the buffer string for assertions."
   "Should error when decoration-char is nil."
   (should-error
    (test-comment-box-at-column 0 ";;" "" nil "Header" 70)
-   :type 'wrong-type-argument))
+   :type 'user-error))
 
 (ert-deftest test-comment-box-elisp-non-integer-length ()
   "Should error when length is not an integer."
diff --git a/tests/test-custom-comments-comment-heavy-box.el b/tests/test-custom-comments-comment-heavy-box.el
index 302896252..94d4aaa5f 100644
--- a/tests/test-custom-comments-comment-heavy-box.el
+++ b/tests/test-custom-comments-comment-heavy-box.el
@@ -207,7 +207,7 @@ Returns the buffer string for assertions."
   "Should error when decoration-char is nil."
   (should-error
    (test-heavy-box-at-column 0 ";;" "" nil "Header" 70)
-   :type 'wrong-type-argument))
+   :type 'user-error))
 
 (ert-deftest test-heavy-box-elisp-nil-text ()
   "Should error when text is nil."
diff --git a/tests/test-custom-comments-comment-inline-border.el b/tests/test-custom-comments-comment-inline-border.el
index ca2bef06c..78e860353 100644
--- a/tests/test-custom-comments-comment-inline-border.el
+++ b/tests/test-custom-comments-comment-inline-border.el
@@ -190,10 +190,12 @@ Returns the buffer string for assertions."
    :type 'error))
 
 (ert-deftest test-inline-border-elisp-nil-decoration ()
-  "Should error when decoration-char is nil."
+  "Should error when decoration-char is nil.
+The decoration-char validator signals `user-error' for any non-printable
+or non-single-character input."
   (should-error
    (test-inline-border-at-column 0 ";;" "" nil "Header" 70)
-   :type 'wrong-type-argument))
+   :type 'user-error))
 
 (ert-deftest test-inline-border-elisp-non-integer-length ()
   "Should error when length is not an integer."
diff --git a/tests/test-custom-comments-comment-padded-divider.el b/tests/test-custom-comments-comment-padded-divider.el
index 702a4c675..d4c189052 100644
--- a/tests/test-custom-comments-comment-padded-divider.el
+++ b/tests/test-custom-comments-comment-padded-divider.el
@@ -200,7 +200,7 @@ Returns the buffer string for assertions."
   "Should error when decoration-char is nil."
   (should-error
    (test-padded-divider-at-column 0 ";;" "" nil "Header" 70 2)
-   :type 'wrong-type-argument))
+   :type 'user-error))
 
 (ert-deftest test-padded-divider-elisp-nil-text ()
   "Should error when text is nil."
diff --git a/tests/test-custom-comments-comment-simple-divider.el b/tests/test-custom-comments-comment-simple-divider.el
index a61e6b4cf..7979f18b7 100644
--- a/tests/test-custom-comments-comment-simple-divider.el
+++ b/tests/test-custom-comments-comment-simple-divider.el
@@ -195,7 +195,7 @@ Returns the buffer string for assertions."
   "Should error when decoration-char is nil."
   (should-error
    (test-simple-divider-at-column 0 ";;" "" nil "Header" 70)
-   :type 'wrong-type-argument))
+   :type 'user-error))
 
 (ert-deftest test-simple-divider-elisp-nil-text ()
   "Should error when text is nil."