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, 76 insertions, 0 deletions

diff --git a/tests/test-system-utils--file-from-context.el b/tests/test-system-utils--file-from-context.el
new file mode 100644
index 00000000..af4b0f82
--- /dev/null
+++ b/tests/test-system-utils--file-from-context.el
@@ -0,0 +1,76 @@
+;;; test-system-utils--file-from-context.el --- Tests for cj/--file-from-context -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `cj/--file-from-context' in system-utils.el.  The
+;; helper returns a file path from the current context, resolving in
+;; priority order: explicit argument, `buffer-file-name', dired file
+;; at point.  Returns nil when none of these yield a file.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'system-utils)
+
+(defmacro test-ffc--with-context (buffer-file in-dired dired-file &rest body)
+  "Run BODY with context stubbed for testing `cj/--file-from-context'.
+BUFFER-FILE becomes the value of `buffer-file-name'.
+IN-DIRED controls `derived-mode-p' (t to simulate dired-mode).
+DIRED-FILE becomes the return value of `dired-file-name-at-point'."
+  (declare (indent 3))
+  `(let ((buffer-file-name ,buffer-file))
+	 (cl-letf (((symbol-function 'derived-mode-p)
+				(lambda (&rest modes)
+				  (and ,in-dired (memq 'dired-mode modes))))
+			   ((symbol-function 'dired-file-name-at-point)
+				(lambda () ,dired-file)))
+	   ,@body)))
+
+;;; Normal cases
+
+(ert-deftest test-ffc-explicit-wins-over-buffer-file ()
+  "Normal: an explicit filename argument wins over `buffer-file-name'."
+  (test-ffc--with-context "/from-buffer.el" nil nil
+	(should (string= "/explicit.el"
+					 (cj/--file-from-context "/explicit.el")))))
+
+(ert-deftest test-ffc-explicit-wins-over-dired ()
+  "Normal: an explicit filename argument wins over dired file at point."
+  (test-ffc--with-context nil t "/from-dired.el"
+	(should (string= "/explicit.el"
+					 (cj/--file-from-context "/explicit.el")))))
+
+(ert-deftest test-ffc-buffer-file-used-when-no-explicit ()
+  "Normal: falls back to `buffer-file-name' when no explicit arg."
+  (test-ffc--with-context "/from-buffer.el" nil nil
+	(should (string= "/from-buffer.el"
+					 (cj/--file-from-context)))))
+
+(ert-deftest test-ffc-dired-used-when-no-explicit-no-buffer-file ()
+  "Normal: in a dired buffer, falls back to dired file at point."
+  (test-ffc--with-context nil t "/from-dired.el"
+	(should (string= "/from-dired.el"
+					 (cj/--file-from-context)))))
+
+;;; Boundary cases
+
+(ert-deftest test-ffc-all-sources-nil-returns-nil ()
+  "Boundary: no explicit, no buffer-file, not in dired → nil."
+  (test-ffc--with-context nil nil nil
+	(should-not (cj/--file-from-context))))
+
+(ert-deftest test-ffc-explicit-nil-uses-fallback-chain ()
+  "Boundary: explicitly passing nil as the arg still uses the fallback chain."
+  (test-ffc--with-context "/from-buffer.el" nil nil
+	(should (string= "/from-buffer.el"
+					 (cj/--file-from-context nil)))))
+
+(ert-deftest test-ffc-dired-mode-but-no-file-at-point ()
+  "Boundary: in dired but nothing at point returns nil (buffer-file also nil)."
+  (test-ffc--with-context nil t nil
+	(should-not (cj/--file-from-context))))
+
+(provide 'test-system-utils--file-from-context)
+;;; test-system-utils--file-from-context.el ends here