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

Comienza con la reproducción vinculada de StackBlitz en src/main.ts y verifica el número de llamadas a childrenAccessor para los nodos expandibles y hoja. Rastrea la implementación de mat-tree responsable de esas llamadas y confirma después que las llamadas solo se producen al expandir y se omiten para los nodos rechazados por el predicado when.

Escrito por el modelo de indexación a partir del texto del issue.