fix: correct shallow copy in router backtracking

The router could panic with a 'slice bounds out of range' error when handling requests that trigger its backtracking logic.

The root cause was a shallow copy of the node's `children` slice when creating a `skippedNode` for backtracking. This could lead to a corrupted state if the router needed to backtrack and then proceed down a wildcard path.

This commit fixes the issue by introducing a `copyChildren` method on the `node` struct, which creates a safe copy of the children slice. This method is now used when creating a `skippedNode`, ensuring that the backtracking logic is isolated and robust.

tree.go

@@ -293,6 +293,12 @@ walk: // 外部循环用于遍历和构建路由树
 	}
 }
 
+func (n *node) copyChildren() []*node {
+	children := make([]*node, len(n.children))
+	copy(children, n.children)
+	return children
+}
+
 // findWildcard 搜索通配符段并检查名称是否包含无效字符。
 // 如果未找到通配符，则返回 -1 作为索引。
 func findWildcard(path string) (wildcard string, i int, valid bool) {
@@ -486,7 +492,7 @@ walk: // 外部循环用于遍历路由树
 									wildChild: n.wildChild,
 									nType:     n.nType,
 									priority:  n.priority,
-									children:  n.children,
+									children:  n.copyChildren(),
 									handlers:  n.handlers,
 									fullPath:  n.fullPath,
 								},