bug(button): overriding not working as expected.

### 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

To override the background color for a button to a particular hex value provided, the state layer needs to be given an opacity of value 1. However, in doing so, the outline is no longer clearly visible. It appears to get hidden behind the state layer/ ::before pseudo element created.

### Reproduction

StackBlitz link: https://stackblitz.com/edit/wwtmsk43?file=src%2Fexample%2Fbutton-overview-example.css

Hover over the outline button to notice the outline no longer being visible.

### Expected Behavior

When opacity for the state layer is set to 1, the state layer for an outlined button should not hide the outline for the button. The outline should still be visible.

### Actual Behavior

The background's stacking context is higher than the button's stacking context which is hiding the border/ outline.

### Environment

- Angular: 20
- CDK/Material: 20

Comienza con la reproducción vinculada de StackBlitz y su src/example/button-overview-example.css; después, inspecciona el comportamiento de apilamiento de state-layer y ::before del botón. Se considera terminado cuando, al pasar el cursor sobre un botón outlined con una opacidad de state-layer de 1, su contorno siga siendo claramente visible y la sobrescritura de fondo hexadecimal proporcionada siga funcionando.

Escrito por el modelo de indexación a partir del texto del issue.