bug(tree): Using cdk-tree with ArrayDataSource and Observable can cause expand functionality to fail under certain conditions.

### Is this a regression?

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

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

_No response_

### Description

Using cdk-tree with ArrayDataSource and Observable, the expand functionality can fail under the following conditions:
1. The Observable emits more than one value asynchronously.
2. The emitted values are dynamically generated.
3. trackBy is not based on identity comparison.

### Reproduction

StackBlitz link: https://stackblitz.com/edit/2ypkpy?file=src%2Fexample%2Fcdk-tree-flat-children-accessor-example.ts
Steps to reproduce:
Click the tree node toggler

### Expected Behavior

Child nodes expand normally.

### Actual Behavior

Child nodes fail to expand.

### Environment

```
_ _ ____ _ ___
/ \ _ __ __ _ _ _| | __ _ _ __ / ___| | |_ _|
/ △ \ | '_ \ / _` | | | | |/ _` | '__| | | | | | |
/ ___ \| | | | (_| | |_| | | (_| | | | |___| |___ | |
/_/ \_\_| |_|\__, |\__,_|_|\__,_|_| \____|_____|___|
|___/

Angular CLI: 18.2.1
Node: 18.20.3
Package Manager: yarn 1.22.19
OS: linux x64

Angular: 18.2.2
... animations, cdk, common, compiler, compiler-cli, core, forms
... localize, material, material-experimental
... material-moment-adapter, platform-browser
... platform-browser-dynamic, router

Package Version
---------------------------------------------------------
@angular-devkit/architect 0.1802.1
@angular-devkit/build-angular 18.2.1
@angular-devkit/core 18.2.1
@angular-devkit/schematics 18.2.1
@angular/cli 18.2.1
@schematics/angular 18.2.1
rxjs 7.4.0
typescript 5.5.4
zone.js 0.14.10
```

- Browser(s): Edge 127.0.2651.105
- Operating System (e.g. Windows, macOS, Ubuntu): Windows 11

リンクされている StackBlitz の再現例、特に src/example/cdk-tree-flat-children-accessor-example.ts から始め、非同期の emission を観察しながらツリーノードのトグラーをクリックします。identity に基づかない trackBy を使用した動的に生成される値に対する cdk-tree の動作を追跡します。この条件で子ノードが正常に展開されれば完了です。

索引モデルが issue の本文から書いたものです。