Fix log cleanup for bare relative log paths

When the log file is configured as a bare file name such as
`ldk-server.log`, its parent is an empty path. Passing that to
`read_dir` fails, so rotated archives were never removed and
`log_max_files` had no effect. Treat an empty parent as the current
working directory, which is where the log file itself is opened.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: benthecarman

ldk-server/src/util/logger.rs

@@ -265,8 +265,14 @@ fn open_log_file(log_file_path: &Path) -> Result<File, io::Error> {
 	OpenOptions::new().create(true).append(true).mode(0o600).open(log_file_path)
 }
 
+/// Returns the directory containing the log file. A bare file name such as `ldk-server.log` has an
+/// empty parent, which refers to the current working directory.
+fn log_file_dir(log_file_path: &Path) -> Option<&Path> {
+	log_file_path.parent().map(|p| if p.as_os_str().is_empty() { Path::new(".") } else { p })
+}
+
 fn cleanup_old_logs(log_file_path: &Path, max_files: usize) -> io::Result<()> {
-	let parent = log_file_path.parent().ok_or_else(|| {
+	let parent = log_file_dir(log_file_path).ok_or_else(|| {
 		io::Error::new(io::ErrorKind::InvalidInput, "Log file path has no parent directory")
 	})?;
 
@@ -344,4 +350,12 @@ mod tests {
 		assert_eq!(mode & 0o077, 0);
 		fs::remove_dir_all(dir).unwrap();
 	}
+
+	#[test]
+	fn log_file_dir_handles_bare_file_name() {
+		assert_eq!(log_file_dir(Path::new("ldk-server.log")), Some(Path::new(".")));
+		assert_eq!(log_file_dir(Path::new("logs/ldk-server.log")), Some(Path::new("logs")));
+		assert_eq!(log_file_dir(Path::new("/var/log/ldk-server.log")), Some(Path::new("/var/log")));
+		assert_eq!(log_file_dir(Path::new("/")), None);
+	}
 }