refactor(external-open): consolidate OS-open dispatch in external-open.el

Phase 4 of utility-consolidation. Three previously-overlapping helpers (system-utils' `cj/identify-external-open-command' and `cj/--open-with-is-launcher-p', plus the dirvish-only `cj/--file-manager-program-for' shipped earlier today) all answered "which OS-open program should I run?".  Pull the answer into one place: external-open.el.

Move and rename:

- `cj/--open-with-is-launcher-p' (system-utils) -> `cj/external-open-launcher-p' (external-open). Public name now matches its module.
- `cj/identify-external-open-command' (system-utils) -> `cj/external-open-command' (external-open). Returns nil for unsupported hosts instead of signaling -- callers that need a command must handle nil explicitly. The wrapper `cj/xdg-open' (also moved into external-open) converts nil to a `user-error' with a clear message, preserving the user-facing failure shape.
- Delete dirvish's `cj/--file-manager-program-for' helper. `cj/dirvish-open-file-manager-here' now calls `cj/external-open-command' directly. The shell-command fallback for nil-program preserves the previous escape hatch.

Break the system-utils <-> external-open recursive require by moving `cj/xdg-open' (the only system-utils function that external-open used) into external-open along with the dispatch.

Tests reorganized to match the move. Two new test files (`test-external-open-command.el', `test-external-open-launcher-p.el') replace the two system-utils-named test files. The dirvish file-manager-program test goes away with the helper. 11 tests covering Normal/Boundary/Error for the dispatch (plus the new "unsupported host returns nil" contract).

Add `(require \='external-open)' to system-utils.el and `(require \='system-lib)' to external-open.el (for `cj/file-from-context' which xdg-open uses).

1 files changed, 55 insertions, 0 deletions

diff --git a/tests/test-external-open-launcher-p.el b/tests/test-external-open-launcher-p.el
new file mode 100644
index 000000000..4aca3a559
--- /dev/null
+++ b/tests/test-external-open-launcher-p.el
@@ -0,0 +1,55 @@
+;;; test-external-open-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.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)
+
+;;; 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-launcher-p)
+;;; test-external-open-launcher-p.el ends here