bug(theming): There is no way to read density scale from theme

### Is this a regression?

- [x] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

18

### Description

In Angular Material 18, developers could use Angular Material's theme's density scale, so that their custom components aligns with other Angular Material components.

In v18, developers could use `mat.get-theme-density($theme);` to get the scale. [Link to docs](https://v18.material.angular.io/guide/theming-your-components#reading-the-density-scale).

In v19, it's not clear how to get density scale from theme.

### Reproduction

Not needed.

### Expected Behavior

Developers should be able to read density scale in Angular Material 19.

### Actual Behavior

Developers are not able to read density scale in Angular Material 19.

### Environment

- Angular: 19
- CDK/Material: 19
- Browser(s): Edge
- Operating System (e.g. Windows, macOS, Ubuntu): Windows

Comienza comparando el punto de entrada “reading the density scale” de la guía de theming de Angular Material v18 con la API de theming de v19. Determina cómo se representa la escala de densidad en el tema de v19 y expón una forma equivalente de leerla para los componentes personalizados. Se considera terminado cuando los desarrolladores puedan recuperar la escala de densidad del tema en Angular Material 19.

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