fix: Add null check for parent directory in S3VolumeManager.downloadFile()

Fixes #212

Added null validation for localPath.getParent() before calling
Files.createDirectories() to prevent NullPointerException in edge cases
where the path has no parent directory.

Changes:
- Store parent path in variable before checking
- Only call Files.createDirectories() if parent is non-null
- Add comment explaining the check

While rare in practice (would require unusual volume path configurations),
this defensive check prevents crashes when dealing with root-level paths
or unusual temporary directory structures.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Flossy

jplatform-storage-s3/src/main/java/org/flossware/jplatform/storage/s3/S3VolumeManager.java

@@ -265,7 +265,12 @@ public void downloadFile(String volumeName, String relativePath) throws IOExcept
         }
 
         Path localPath = getVolumePath(volumeName).resolve(relativePath);
-        Files.createDirectories(localPath.getParent());
+
+        // Create parent directories if parent exists
+        Path parent = localPath.getParent();
+        if (parent != null) {
+            Files.createDirectories(parent);
+        }
 
         try {
             String s3Key = buildS3Key(volumeName, relativePath);