bug(cdkTable & virtual scroll): Virtual scrolling in CDK table provides incorrect information to screen readers and causes focus/context loss during navigation

### 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

The recently introduced virtual scrolling support for CDK tables appears to have accessibility issues when used with screen readers.

Because rows are dynamically removed and added to the DOM, screen reader users can receive incorrect table information and lose their position while navigating through table content.

### Impact

This issue significantly affects accessibility for users who rely on screen readers and may make large virtualized tables difficult or impossible to navigate reliably.

### Reproduction

https://github.com/user-attachments/assets/e2afd2cd-d439-4be2-815b-47208ac93e5e

https://rc.material.angular.dev/cdk/table/overview#alternate-html-to-using-native-table
Steps to reproduce:
1. Start a screen reader (e.g. NVDA, JAWS, or VoiceOver).
2. Navigate to the table and begin reading/navigating through rows using screen reader table navigation commands.
3. Continue navigating down the table until virtual scrolling is triggered and previously rendered rows are removed from the DOM while new rows are rendered.

### Expected Behavior

Screen readers should be able to determine the actual total number of rows in the table, not only the rows currently rendered in the DOM.
Navigation through table rows should remain stable when virtual scrolling updates the rendered range.
Users should not unexpectedly jump to different data records while their screen reader focus remains on the same DOM row.
The virtual scrolling implementation should preserve a consistent and accessible experience for assistive technology users.

### Actual Behavior

Current Behavior

### Issue 1: Incorrect row count

Screen readers report an incorrect number of rows in the table.

Since only a subset of rows is rendered at any given time, assistive technologies may perceive only the currently rendered rows instead of the actual total number of rows in the dataset.

For example, a table containing thousands of rows may be announced as having only the number of rows currently present in the DOM.

### Issue 2: Row content changes while screen reader focus remains on the same row

When navigating the table with a screen reader, virtual scrolling can remove rows from the beginning of the DOM and append new rows at the end.

**Example:**

The user navigates down to row 30.
Virtual scrolling is triggered.
Rows 1–10 are removed from the DOM.
Rows 31–40 are added.
The screen reader remains positioned on what it considers "row 30".

However, the DOM element representing that row may now contain different content than before. As a result, the user effectively jumps to another record without receiving clear feedback that the underlying content has changed.

This makes navigation confusing and can cause users to lose context within the table.

### Environment

- Angular: 22
- CDK/Material: RC
- Browser(s): Chrome 149.0.7827.103
- Operating System (e.g. Windows, macOS, Ubuntu): macOS 26.5.1 (25F80)

从 CDK table virtual-scrolling 重现和链接的 CDK table overview 示例开始，在移除和渲染行时使用屏幕阅读器。调查错误的行总数，以及导航期间焦点或上下文的变化。当屏幕阅读器收到实际行数，并且在渲染范围更新时导航仍保持稳定，即表示完成。

由索引模型根据 Issue 内容生成。