bug(cdk-tree): CDK Tree can't be opened when trackBy function is used after update of datasource

### Is this a regression?

- [x] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

Angular 17 (here is the same example: https://stackblitz.com/edit/zmfrj6ks?file=src%2Fexample%2Fcdk-tree-nested-example.ts,src%2Fexample%2Fcdk-tree-nested-example.html with Angular 17)

### Description

Hi, there is a bug in the Angular CDK Tree. When you update the data of the tree and a trackBy function is given to the tree, you can't interact with the tree anymore.

### Reproduction

StackBlitz link: https://stackblitz.com/edit/fesfbplt?file=src%2Fexample%2Fcdk-tree-nested-children-accessor-example.html
Steps to reproduce:
1. interact with the tree (not mandatory, but you see that it works)
2. click on "changeTree" button
3. try to open the tree again => this doesn't work

### Expected Behavior

Tree should be updateable and interactive after an update. I would also expect that the open state is kept when only part of the tree is changed (in the example one subtree is removed, the other persists) and a trackby function is given.

### Actual Behavior

Tree is closed and not interactive anymore.

### Environment

- Angular: 18/19
- CDK/Material: 18/19
- Browser(s): all
- Operating System (e.g. Windows, macOS, Ubuntu): windows

从链接的 StackBlitz 复现开始，研究 Angular CDK Tree 围绕其 trackBy 函数和 datasource 更新的行为。复现 changeTree 序列，然后跟踪参与数据更新和保留交互状态的 CDK Tree 入口点。完成的标准是：树在更新后仍可交互，并保留未更改子树的打开状态。

由索引模型根据 Issue 内容生成。