bug(mat-tree): childrenAccessor is not lazily evaluated and leaf nodes are not filtered out before calling the childrenAccessor

### 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

`childrenAccessor` should only be called when expanding a node.

In the current state, workarounds are needed to support a mat-tree which asynchronously fetches children. If the dataset is large enough, there will then be a proportionately large number of fetches for the children.

### Reproduction

StackBlitz link: [stackblitz](https://stackblitz.com/edit/angular18-tree-duplicates-6iwee3dq?file=src%2Fmain.ts)

Notice how the tree eagerly fetches all children, including children of leaf nodes. The number on the bottom of the page indicates the number of times childrenAccessor was called.

### Expected Behavior

`childrenAccessor` should only be called when expanding a node.

Moreover, it should also *not* call the childrenAccessor for nodes which are not expandable, since we already provide the `when: hasChild` predicate.

### Actual Behavior

`childrenAccessor` is eagerly called for each node when the tree is created, including leaf nodes.

### Environment

- Angular: 20
- CDK/Material: 20
- Browser(s): Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): MacOS

Inizia con la riproduzione StackBlitz collegata in src/main.ts e verifica il numero di chiamate a childrenAccessor per i nodi espandibili e foglia. Traccia l’implementazione di mat-tree responsabile di queste chiamate, quindi conferma che le chiamate avvengono solo durante l’espansione e vengono ignorate per i nodi rifiutati dal predicato when.

Scritto dal modello di indicizzazione a partire dal testo della issue.