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

Start with the linked StackBlitz reproduction at src/main.ts and verify the childrenAccessor call count for expandable and leaf nodes. Trace the mat-tree implementation responsible for those calls, then confirm that calls occur only on expansion and are skipped for nodes rejected by the when predicate.

Written by the indexing model from the issue text.