refactor(system-utils): extract testable open-file helpers

Extracts two pure helpers from cj/open-file-with-command and cj/xdg-open so the file-resolution and launcher-detection logic becomes testable without mocking process launchers.

New helpers:

- cj/--file-from-context returns a file path from the current context, resolving in priority order (explicit arg, buffer-file-name, dired file at point). Returns nil when none apply.
- cj/--open-with-is-launcher-p is a predicate for whether a command is a desktop launcher (xdg-open, open, start) that needs call-process detachment.

Both commands now delegate. cj/open-file-with-command uses cj/--file-from-context with read-file-name as the final fallback, plus cj/--open-with-is-launcher-p for the launcher dispatch. cj/xdg-open uses cj/--file-from-context with user-error as the "no file" fallback.

Behavior preserved. The existing system-utils test suites still pass, and the shape of each command's final effect is identical.

New tests, 14 cases across two per-function files:

- tests/test-system-utils--file-from-context.el covers: explicit wins over buffer-file, explicit wins over dired, buffer-file fallback, dired fallback, all-nil returns nil, explicit-nil uses chain, dired-mode-but-no-file-at-point.
- tests/test-system-utils--open-with-is-launcher-p.el covers: each of the three launcher names returns t, non-launcher returns nil, empty string returns nil, case-sensitive check, nil input returns nil.

Coverage: system-utils.el went from 10/52 (19.2%) to 15/52 (28.8%). The remaining uncovered lines are mostly in the process-launching paths of cj/open-file-with-command and cj/xdg-open. Those are testability-blocked. Mocking call-process, start-process-shell-command, and generate-new-buffer would give a lot of mock surface for low value. cj/server-shutdown is not meaningfully testable because it kills Emacs.

1 files changed, 52 insertions, 0 deletions

diff --git a/tests/test-system-utils--open-with-is-launcher-p.el b/tests/test-system-utils--open-with-is-launcher-p.el
new file mode 100644
index 000000000..64e9a4b6f
--- /dev/null
+++ b/tests/test-system-utils--open-with-is-launcher-p.el
@@ -0,0 +1,52 @@
+;;; test-system-utils--open-with-is-launcher-p.el --- Tests for cj/--open-with-is-launcher-p -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `cj/--open-with-is-launcher-p' in system-utils.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)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'system-utils)
+
+;;; Normal cases
+
+(ert-deftest test-owilp-xdg-open-is-launcher ()
+  "Normal: \"xdg-open\" (Linux launcher) returns t."
+  (should (eq t (cj/--open-with-is-launcher-p "xdg-open"))))
+
+(ert-deftest test-owilp-open-is-launcher ()
+  "Normal: \"open\" (macOS launcher) returns t."
+  (should (eq t (cj/--open-with-is-launcher-p "open"))))
+
+(ert-deftest test-owilp-start-is-launcher ()
+  "Normal: \"start\" (Windows launcher) returns t."
+  (should (eq t (cj/--open-with-is-launcher-p "start"))))
+
+;;; Boundary cases
+
+(ert-deftest test-owilp-non-launcher-command-returns-nil ()
+  "Boundary: a non-launcher command (e.g. gimp) returns nil."
+  (should-not (cj/--open-with-is-launcher-p "gimp")))
+
+(ert-deftest test-owilp-empty-string-returns-nil ()
+  "Boundary: empty string is not a launcher."
+  (should-not (cj/--open-with-is-launcher-p "")))
+
+(ert-deftest test-owilp-case-sensitive ()
+  "Boundary: launcher check is case-sensitive (\"Open\" is not \"open\")."
+  (should-not (cj/--open-with-is-launcher-p "Open"))
+  (should-not (cj/--open-with-is-launcher-p "XDG-OPEN")))
+
+;;; Error cases
+
+(ert-deftest test-owilp-nil-argument-returns-nil ()
+  "Error: nil input is handled gracefully (not in the launcher list)."
+  (should-not (cj/--open-with-is-launcher-p nil)))
+
+(provide 'test-system-utils--open-with-is-launcher-p)
+;;; test-system-utils--open-with-is-launcher-p.el ends here