feat:system: Add system utility library with executable check

Introduce a new `system-lib.el` module providing low-level system
utility functions, including function `cj/executable-exists-p` to
check for the availability of programs in PATH. Integrate this
library in `init.el`.

test(system): Add unit tests for executable check function

Create comprehensive unit tests for `cj/executable-exists-p` in
`system-lib.el`, ensuring coverage of normal, boundary and error
scenarios.

1 files changed, 73 insertions, 0 deletions

diff --git a/tests/test-system-lib-executable-exists-p.el b/tests/test-system-lib-executable-exists-p.el
new file mode 100644
index 00000000..457bb010
--- /dev/null
+++ b/tests/test-system-lib-executable-exists-p.el
@@ -0,0 +1,73 @@
+;;; test-system-lib-executable-exists-p.el --- Tests for cj/executable-exists-p -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for cj/executable-exists-p function from system-lib.el.
+;; Tests whether external programs are correctly detected in PATH.
+
+;;; Code:
+
+(require 'ert)
+(require 'system-lib)
+
+;;; Normal Cases
+
+(ert-deftest test-system-lib-executable-exists-p-normal-existing-program-returns-path ()
+  "Test that existing program in PATH returns non-nil.
+
+Standard case: checking for a program that definitely exists on all systems."
+  (should (cj/executable-exists-p "ls")))
+
+(ert-deftest test-system-lib-executable-exists-p-normal-diff-exists-returns-path ()
+  "Test that diff program exists and is detected.
+
+Tests specifically for diff which we use in our diff functionality."
+  (should (cj/executable-exists-p "diff")))
+
+;;; Boundary Cases
+
+(ert-deftest test-system-lib-executable-exists-p-boundary-empty-string-returns-nil ()
+  "Test that empty string returns nil.
+
+Boundary case: empty string is not a valid program name."
+  (should-not (cj/executable-exists-p "")))
+
+(ert-deftest test-system-lib-executable-exists-p-boundary-whitespace-only-returns-nil ()
+  "Test that whitespace-only string returns nil.
+
+Boundary case: strings containing only whitespace are not valid programs."
+  (should-not (cj/executable-exists-p "   ")))
+
+(ert-deftest test-system-lib-executable-exists-p-boundary-absolute-path-returns-path ()
+  "Test that absolute path to executable returns the path.
+
+Boundary case: executable-find accepts both program names and full paths."
+  (should (cj/executable-exists-p "/usr/bin/ls")))
+
+;;; Error Cases
+
+(ert-deftest test-system-lib-executable-exists-p-error-nil-input-returns-nil ()
+  "Test that nil input returns nil gracefully.
+
+Error case: nil is not a valid program name."
+  (should-not (cj/executable-exists-p nil)))
+
+(ert-deftest test-system-lib-executable-exists-p-error-number-input-returns-nil ()
+  "Test that numeric input returns nil gracefully.
+
+Error case: number is not a valid program name."
+  (should-not (cj/executable-exists-p 42)))
+
+(ert-deftest test-system-lib-executable-exists-p-error-nonexistent-program-returns-nil ()
+  "Test that nonexistent program returns nil.
+
+Error case: program that definitely doesn't exist in PATH."
+  (should-not (cj/executable-exists-p "this-program-definitely-does-not-exist-xyz123")))
+
+(ert-deftest test-system-lib-executable-exists-p-error-special-characters-returns-nil ()
+  "Test that program name with special characters returns nil.
+
+Error case: invalid characters in program name."
+  (should-not (cj/executable-exists-p "program-with-$pecial-ch@rs")))
+
+(provide 'test-system-lib-executable-exists-p)
+;;; test-system-lib-executable-exists-p.el ends here