refactor(external-open): extract external-open-lib for shared helpers

Same shared-helpers split-pattern that ai-vterm/vterm-config use
through cj-window-toggle-lib and that calendar-sync uses through
cj-org-text-lib.  Pull the two pure dispatch helpers out of the
external-open feature module into a sibling library so consumers
that only need the dispatch don't have to require the whole feature.

New `modules/external-open-lib.el' carries:
- `cj/external-open-command'
- `cj/external-open-launcher-p'

`modules/external-open.el' stays as the feature module: the
`default-open-extensions' defcustom, the `find-file' advice
(`cj/find-file-auto'), and the interactive commands (`cj/xdg-open',
`cj/open-this-file-with').  It now requires external-open-lib for
the dispatch helpers.

Migrate consumers:
- system-utils.el used to require `external-open' for
  `cj/external-open-launcher-p' alone -- now requires
  `external-open-lib' directly.
- dirvish-config.el calls `cj/external-open-command' from
  `cj/dirvish-open-file-manager-here' -- add an explicit
  `(require \='external-open-lib)'.

Test files renamed to match the system-lib naming pattern
(test-<library>-<feature>.el):
- test-external-open-command.el -> test-external-open-lib-command.el
- test-external-open-launcher-p.el -> test-external-open-lib-launcher-p.el

No behavior change.

1 files changed, 55 insertions, 0 deletions

diff --git a/tests/test-external-open-lib-launcher-p.el b/tests/test-external-open-lib-launcher-p.el
new file mode 100644
index 00000000..928293b4
--- /dev/null
+++ b/tests/test-external-open-lib-launcher-p.el
@@ -0,0 +1,55 @@
+;;; test-external-open-lib-launcher-p.el --- Tests for cj/external-open-launcher-p -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `cj/external-open-launcher-p' in external-open-lib.el.
+;; The predicate returns t for desktop launcher commands (xdg-open,
+;; open, start) that need `call-process' with a zero buffer argument
+;; to fully detach from Emacs.  Anything else returns nil.
+
+;;; Code:
+
+(require 'ert)
+(require 'package)
+
+(setq package-user-dir (expand-file-name "elpa" user-emacs-directory))
+(package-initialize)
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'external-open-lib)
+
+;;; Normal cases
+
+(ert-deftest test-eolp-xdg-open-is-launcher ()
+  "Normal: \"xdg-open\" (Linux launcher) returns t."
+  (should (eq t (cj/external-open-launcher-p "xdg-open"))))
+
+(ert-deftest test-eolp-open-is-launcher ()
+  "Normal: \"open\" (macOS launcher) returns t."
+  (should (eq t (cj/external-open-launcher-p "open"))))
+
+(ert-deftest test-eolp-start-is-launcher ()
+  "Normal: \"start\" (Windows launcher) returns t."
+  (should (eq t (cj/external-open-launcher-p "start"))))
+
+;;; Boundary cases
+
+(ert-deftest test-eolp-non-launcher-command-returns-nil ()
+  "Boundary: a non-launcher command (e.g. gimp) returns nil."
+  (should-not (cj/external-open-launcher-p "gimp")))
+
+(ert-deftest test-eolp-empty-string-returns-nil ()
+  "Boundary: empty string is not a launcher."
+  (should-not (cj/external-open-launcher-p "")))
+
+(ert-deftest test-eolp-case-sensitive ()
+  "Boundary: launcher check is case-sensitive (\"Open\" is not \"open\")."
+  (should-not (cj/external-open-launcher-p "Open"))
+  (should-not (cj/external-open-launcher-p "XDG-OPEN")))
+
+;;; Error cases
+
+(ert-deftest test-eolp-nil-argument-returns-nil ()
+  "Error: nil input is handled gracefully (not in the launcher list)."
+  (should-not (cj/external-open-launcher-p nil)))
+
+(provide 'test-external-open-lib-launcher-p)
+;;; test-external-open-lib-launcher-p.el ends here