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

Start with the mat-button disabled-appearance styles and the CSS selector that currently checks only [disabled]. Reproduce the issue using the linked StackBlitz with buttons inside ; done means both buttons have the same disabled appearance, including the explicitly disabled button case.

Written by the indexing model from the issue text.