test(system-utils): cover identify-external-open-command and eval-buffer

Adds two per-function test files covering the easily-testable functions in system-utils.el. Takes the module from 0/52 (0%) to 10/52 (19.2%) coverage.

tests/test-system-utils-identify-external-open-command.el — five tests for cj/identify-external-open-command: one each for the three supported platforms (linux, macos, windows), one documenting the dispatch-order invariant when multiple predicates return t, and one for the unsupported-host error case. env-*-p predicates are stubbed via cl-letf so the tests run correctly on any host.

tests/test-system-utils-eval-buffer.el — three tests for cj/eval-buffer-with-confirmation-or-error-message: the valid-elisp success path, an empty-buffer boundary case, and the broken-elisp error path (confirming the condition-case wrapper catches errors rather than propagating). A small macro stubs message to capture what the user would see.

Split into two files per the project's per-function test convention (elisp-testing.md).

Not covered in this commit and flagged for follow-up:

- cj/open-file-with-command and cj/xdg-open mix user-input resolution with process launching. Per elisp-testing.md's "split interactive from internal" rule, a pure file-resolution helper should be extracted before tests are written.
- cj/server-shutdown kills Emacs, so it isn't meaningfully testable.

1 files changed, 59 insertions, 0 deletions

diff --git a/tests/test-system-utils-identify-external-open-command.el b/tests/test-system-utils-identify-external-open-command.el
new file mode 100644
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--- /dev/null
+++ b/tests/test-system-utils-identify-external-open-command.el
@@ -0,0 +1,59 @@
+;;; test-system-utils-identify-external-open-command.el --- Tests for cj/identify-external-open-command -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `cj/identify-external-open-command' in system-utils.el.
+;; The function dispatches on host-environment predicates to return the
+;; appropriate "open" command: xdg-open on Linux, open on macOS,
+;; start on Windows.  Anything else is a fatal error.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'system-utils)
+
+(defmacro test-siuec--with-host (linux macos windows &rest body)
+  "Run BODY with env-*-p predicates stubbed to LINUX, MACOS, WINDOWS."
+  (declare (indent 3))
+  `(cl-letf (((symbol-function 'env-linux-p)   (lambda () ,linux))
+			 ((symbol-function 'env-macos-p)   (lambda () ,macos))
+			 ((symbol-function 'env-windows-p) (lambda () ,windows)))
+	 ,@body))
+
+;;; Normal cases
+
+(ert-deftest test-siuec-linux-returns-xdg-open ()
+  "Normal: Linux host returns \"xdg-open\"."
+  (test-siuec--with-host t nil nil
+	(should (string= "xdg-open" (cj/identify-external-open-command)))))
+
+(ert-deftest test-siuec-macos-returns-open ()
+  "Normal: macOS host returns \"open\"."
+  (test-siuec--with-host nil t nil
+	(should (string= "open" (cj/identify-external-open-command)))))
+
+(ert-deftest test-siuec-windows-returns-start ()
+  "Normal: Windows host returns \"start\"."
+  (test-siuec--with-host nil nil t
+	(should (string= "start" (cj/identify-external-open-command)))))
+
+;;; Boundary cases
+
+(ert-deftest test-siuec-dispatch-order-linux-wins ()
+  "Boundary: Linux check runs first; wins when predicates disagree.
+Documents the dispatch order.  A real host only returns t from one
+of these anyway — but if something goes wrong, Linux takes priority."
+  (test-siuec--with-host t t t
+	(should (string= "xdg-open" (cj/identify-external-open-command)))))
+
+;;; Error cases
+
+(ert-deftest test-siuec-unsupported-host-signals-error ()
+  "Error: when no platform predicate returns non-nil, signals an error."
+  (test-siuec--with-host nil nil nil
+	(should-error (cj/identify-external-open-command))))
+
+(provide 'test-system-utils-identify-external-open-command)
+;;; test-system-utils-identify-external-open-command.el ends here