bug(mat-button): mat-button should be changed to disabled appearance when the parent `<fieldset>` is disabled

### 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 same issue #3033 was closed by angular/angular#35309. But it only fixes form controls not buttons.

The CSS selector for disabled appearance only selects `[disabled]` but it should also select `:disabled` to make the parent `` work.

### Reproduction

StackBlitz link: https://stackblitz.com/edit/components-issue-starter-ck6fmjkd
Steps to reproduce:
1. Put both `mat-button` and **disabled**-`mat-button` in ``.

### Expected Behavior

Both `mat-button` should show the same appearance.

### Actual Behavior

They are different.

### Environment

- Angular: 20.0.6
- CDK/Material: 20.0.5
- Browser(s): Google Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): Windows

mat-button の無効状態の外観スタイルと、現在 [disabled] のみをチェックしている CSS セレクターから始めます。 内にボタンを配置した、リンク先の StackBlitz を使って問題を再現します。両方のボタンが同じ無効状態の外観になり、明示的に無効化されたボタンの場合も含まれることが完了条件です。

索引モデルが issue の本文から書いたものです。