bug(virtual-scroll): scrollToIndex on viewport while inside cdkVirtualScrollingElement is wrong

### 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

When scrolling by index using `scrollToIndex` on the viewport it doesn't take into account any content around the viewport when using `cdkVirtualScrollingElement`.
The logic calculates the offset only based on the `itemSize` without taking into account the viewport offset.

### Reproduction

StackBlitz link: https://stackblitz.com/edit/qmx1v9bc
Steps to reproduce:
1. In the `Regular` section clicking on the button `Scroll to item 50` correctly scrolls to item 50. This is a regular viewport without `cdkVirtualScrollingElement`.
2. In the `With cdkVirtualScrollingElement` section clicking on the button `Scroll to item 50 (not working)` it scrolls to item 48 instead of 50 as there is a 100px header above the viewport while it should scroll to item 50.
3. If the button `Scroll to item 50 (fixed)` is clicked the scrolling works as expected as I calculate the offset myself using `viewport.measureViewportOffset('top')` which is missing in `scrollToIndex`.

### Expected Behavior

It should scroll to the correct item when using `cdkVirtualScrollingElement`

### Actual Behavior

It scrolls without taking into account the viewport offset.

### Environment

- Angular: 21.2.6
- CDK/Material: 21.1.2
- Browser(s): Chrome 146
- Operating System (e.g. Windows, macOS, Ubuntu): Windows 11

Bắt đầu với phần triển khai viewport cuộn ảo của CDK và entry point scrollToIndex, sau đó so sánh hành vi của nó với measureViewportOffset('top') trong bản tái hiện StackBlitz được cung cấp. Xác minh thay đổi với các trường hợp regular viewport và cdkVirtualScrollingElement: cuộn đến mục 50 phải chọn mục 50 trong cả hai phần, kể cả khi có offset 100px của header.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.