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/empty_folder_cleanup/empty_folder_cleaner.go

@@ -27,6 +27,7 @@ type FilerOperations interface {
 	CountDirectoryEntries(ctx context.Context, dirPath util.FullPath, limit int) (count int, err error)
 	DeleteEntryMetaAndData(ctx context.Context, p util.FullPath, isRecursive, ignoreRecursiveError, shouldDeleteChunks, isFromOtherCluster bool, signatures []int32, ifNotModifiedAfter int64) error
 	GetEntryAttributes(ctx context.Context, p util.FullPath) (attributes map[string][]byte, err error)
+	IsDirectoryKeyObject(ctx context.Context, p util.FullPath) (bool, error)
 }
 
 // folderState tracks the state of a folder for empty folder cleanup
@@ -312,6 +313,16 @@ func (efc *EmptyFolderCleaner) executeCleanup(folder string, triggeredBy string)
 		return
 	}
 
+	// Skip explicitly created directory markers (e.g., PUT /bucket/folder/)
+	// These have a MIME type set and should be preserved even when empty
+	if isKeyObj, err := efc.filer.IsDirectoryKeyObject(ctx, util.FullPath(folder)); err != nil {
+		glog.V(2).Infof("EmptyFolderCleaner: error checking directory key object %s: %v", folder, err)
+		return
+	} else if isKeyObj {
+		glog.V(3).Infof("EmptyFolderCleaner: skipping %s (triggered by %s), explicit directory marker", folder, triggeredBy)
+		return
+	}
+
 	// Delete the empty folder
 	glog.Infof("EmptyFolderCleaner: deleting empty folder %s (triggered by %s)", folder, triggeredBy)
 	if err := efc.deleteFolder(ctx, folder); err != nil {