fix(s3): preserve explicit directory markers during empty folder cleanup (#8831)

* fix(s3): preserve explicit directory markers during empty folder cleanup

PR #8292 switched empty-folder cleanup from per-folder implicit checks
to bucket-level policy, inadvertently dropping the check that preserved
explicitly created directories (e.g., PUT /bucket/folder/). This caused
user-created folders to be deleted when their last file was removed.

Add IsDirectoryKeyObject check in executeCleanup to skip folders that
have a MIME type set, matching the canonical pattern used throughout the
S3 listing and delete handlers.

* fix: handle ErrNotFound in IsDirectoryKeyObject for race safety

Entry may be deleted between the emptiness check and the directory
marker lookup. Treat not-found as false rather than propagating
the error, avoiding unnecessary error logging in the cleanup path.

* refactor: consolidate directory marker tests and tidy error handling

- Combine two separate test functions into a table-driven test
- Nest ErrNotFound check inside the err != nil block

weed/filer/filer.go

@@ -559,3 +559,17 @@ func (f *Filer) GetEntryAttributes(ctx context.Context, p util.FullPath) (map[st
 	}
 	return entry.Extended, nil
 }
+
+func (f *Filer) IsDirectoryKeyObject(ctx context.Context, p util.FullPath) (bool, error) {
+	entry, err := f.FindEntry(ctx, p)
+	if err != nil {
+		if errors.Is(err, filer_pb.ErrNotFound) {
+			return false, nil
+		}
+		return false, err
+	}
+	if entry == nil {
+		return false, nil
+	}
+	return entry.IsDirectory() && entry.Mime != "", nil
+}