fix: delete folders using postorder (#113493)

* fix: delete folders using postorder

* chore: use helper function and do not add method to Folder store

- addresses other review comments fixing log messages and cleans up the unit tests

* chore: run library element tests on modes 2,3,5 only

* chore: adjust to folder.SortByPostorder(folders []*Folder)

* chore: run library panels tests in mode 2,3,5 only

* chore: run tests in all modes and increase timeout

- adjusting the modes and tweaking configs will be done separately

pkg/services/folder/model.go

@@ -278,3 +278,50 @@ type GetDescendantCountsQuery struct {
 }
 
 type DescendantCounts map[string]int64
+
+// SortByPostorder returns the folders in postorder traversal order.
+// That is, children folders appear before their parents in the returned slice.
+func SortByPostorder(folders []*Folder) []*Folder {
+	if len(folders) == 0 {
+		return folders
+	}
+
+	// Build parent-to-children map
+	tree := make(map[string][]*Folder)
+	folderMap := make(map[string]*Folder)
+	for _, f := range folders {
+		folderMap[f.UID] = f
+		tree[f.ParentUID] = append(tree[f.ParentUID], f)
+	}
+
+	// Find all roots (folders whose parents are not in the result set)
+	var roots []*Folder
+	for _, f := range folders {
+		if folderMap[f.ParentUID] == nil {
+			roots = append(roots, f)
+		}
+	}
+
+	// Traverse in postorder
+	result := make([]*Folder, 0, len(folders))
+	visited := make(map[string]bool)
+	var traverse func(f *Folder)
+	traverse = func(f *Folder) {
+		if visited[f.UID] {
+			return
+		}
+		visited[f.UID] = true
+		// First visit all children
+		for _, child := range tree[f.UID] {
+			traverse(child)
+		}
+		// Then add current folder
+		result = append(result, f)
+	}
+
+	for _, root := range roots {
+		traverse(root)
+	}
+
+	return result
+}