bug(cdk/text-field/autosize): incorrect height calculation when parent container adds a scrollbar due to textarea overflow

### 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 a `` with `cdkTextareaAutosize` has its **width set to** `auto` and sits flush against the edges of its parent container, `autosize` miscalculates its height **when the parent container acquires a vertical scrollbar** (typically when overflow-y: auto or scroll is set).

As soon as the parent container shows a vertical scrollbar - slightly reducing the effective available width - the autosize clone measures text wrapping differently, causing the textarea to underestimate the required number of rows. This results in **clipped and partially hidden text**.

### Reproduction

StackBlitz link: **https://stackblitz.com/edit/5xmbl7sj-bxb7txwe?file=src%2Fexample%2Ftext-field-autosize-textarea-example.html**
Steps to reproduce:
1. Open the StackBlitz Link above
2. Observe the `<div>` container with `overflow-y: auto` and the `<textarea>` component using `cdkTextareaAutosize`
3. Resize the viewport so the container becomes narrow enough to easily trigger a scrollbar
4. Insert text until the text area is filled and looks something like this, such that the scrollbar is triggered

<img width="309" height="249" alt="Image" src="https://github.com/user-attachments/assets/bd4bd212-e4e0-40b2-98bb-1ff800d20b33" />

5. Insert another letter like `a`, note how it disappears
6. Use the cursor to navigate to the bottom of the text and note how the top of the text in the `textarea` is clipping.

### Expected Behavior

`cdkTextareaAutosize` should correctly measure the content height **even when the parent container introduces a scrollbar** that slightly changes the available width. The textarea should always fully display its content without clipping or requiring manual resizing.

### Actual Behavior

When the container’s scrollbar appears, the autosize clone measures text wrapping as if no scrollbar were present. This results in a mismatch between measured and actual layout width, underestimating the required height by roughly one line. The resulting height is underestimated by roughly one line, leading to visible clipping.

<img width="304" height="292" alt="Image" src="https://github.com/user-attachments/assets/70026306-40ba-4766-83ad-9cfc3c6fb5e5" />

### Environment

- Angular: `20.2.4`
- CDK/Material: `20.2.2`
- Browser(s): `Edge (141.0.3537.85), Chrome (141.0.7390.108)` possibly more (tested on these)
- Operating System (e.g. Windows, macOS, Ubuntu): `Windows`

src/example/text-field-autosize-textarea-example.html にあるリンク先の StackBlitz 例から始め、親要素のスクロールバーを発生させて問題を再現します。次に cdkTextareaAutosize の実装を追い、クローンの測定値と textarea の幅を比較します。報告されている Angular および CDK のバージョンで、スクロールバーによって利用可能な幅が変化してもテキストが完全に表示され続ければ完了です。

索引モデルが issue の本文から書いたものです。