fixes h2oaiGH-16468

Author: hutch3232

h2o-persist-s3/src/main/java/water/persist/PersistS3.java

@@ -237,6 +237,24 @@ public PersistEntry[] list(String path) {
     return entries;
   }
 
+  @Override
+  public String getParent(String path) {
+    String[] bk = decodePath(path);
+    String bucket = bk[0];
+    String key = bk[1];
+    if (key.endsWith("/")) {
+      key = key.substring(0, key.length() - 1);
+    }
+    if (key == null || key.isEmpty()) {
+      throw new IllegalArgumentException("No parent exists for a bucket-only path: " + path);
+    }
+    int lastSlash = key.lastIndexOf("/");
+    if (lastSlash == -1) {
+      return "s3://" + bucket;
+    }
+    return "s3://" + bucket + "/" + key.substring(0, lastSlash);
+  }
+
   @Override
   public InputStream open(String path) {
     String[] bk = decodePath(path);