test: backfill edge cases for notify path helpers

Cover expand_tilde (bare ~, ~/sub, absolute, relative, non-leading tilde,
~user, whitespace) and validate_custom_path (empty/whitespace, missing,
existing file vs dir via CARGO_MANIFEST_DIR, tilde-prefixed missing).
Test-only; no production code or dependency changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/notify.rs

@@ -170,18 +170,42 @@ mod tests {
     #[test]
     fn test_expand_tilde() {
         if let Some(home) = dirs::home_dir() {
+            // bare `~` -> home dir
             assert_eq!(expand_tilde("~"), home);
-            assert_eq!(expand_tilde("~/foo/bar.wav"), home.join("foo/bar.wav"));
+            // `~/sub/dir/file.wav` -> home joined with `sub/dir/file.wav`
+            assert_eq!(expand_tilde("~/sub/dir/file.wav"), home.join("sub/dir/file.wav"));
         }
-        assert_eq!(expand_tilde("/absolute/path.wav"), PathBuf::from("/absolute/path.wav"));
-        assert_eq!(expand_tilde("relative/path.wav"), PathBuf::from("relative/path.wav"));
-        assert_eq!(expand_tilde("   "), PathBuf::from(""));
+        // absolute path (`/etc/foo.wav`) -> unchanged
+        assert_eq!(expand_tilde("/etc/foo.wav"), PathBuf::from("/etc/foo.wav"));
+        // relative path (`foo/bar.wav`) -> unchanged
+        assert_eq!(expand_tilde("foo/bar.wav"), PathBuf::from("foo/bar.wav"));
+        // a tilde not at the start (`/opt/~/x.wav`) -> unchanged
+        assert_eq!(expand_tilde("/opt/~/x.wav"), PathBuf::from("/opt/~/x.wav"));
+        // `~username` style (`~bob/x.wav`) -> unchanged
+        assert_eq!(expand_tilde("~bob/x.wav"), PathBuf::from("~bob/x.wav"));
+        // leading/trailing whitespace is trimmed
+        if let Some(home) = dirs::home_dir() {
+            assert_eq!(expand_tilde("  ~  "), home);
+            assert_eq!(expand_tilde("  ~/sub/dir/file.wav  "), home.join("sub/dir/file.wav"));
+        }
+        assert_eq!(expand_tilde("  /etc/foo.wav  "), PathBuf::from("/etc/foo.wav"));
+        assert_eq!(expand_tilde("  foo/bar.wav  "), PathBuf::from("foo/bar.wav"));
     }
 
     #[test]
     fn test_validate_custom_path() {
+        // empty and whitespace-only -> None
         assert_eq!(validate_custom_path(""), None);
         assert_eq!(validate_custom_path("   "), None);
+        // a path that does not exist -> Some(false)
         assert_eq!(validate_custom_path("/non/existent/file.wav"), Some(false));
+        // a real existing file -> Some(true)
+        let existing_file = concat!(env!("CARGO_MANIFEST_DIR"), "/Cargo.toml");
+        assert_eq!(validate_custom_path(existing_file), Some(true));
+        // an existing directory -> Some(false)
+        let existing_dir = env!("CARGO_MANIFEST_DIR");
+        assert_eq!(validate_custom_path(existing_dir), Some(false));
+        // a `~`-prefixed path that resolves to a non-existent file -> Some(false)
+        assert_eq!(validate_custom_path("~/non/existent/file.wav"), Some(false));
     }
 }