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

Inizia confrontando il punto di ingresso “reading the density scale” della guida al theming di Angular Material v18 con l’API di theming della v19. Determina come viene rappresentata la scala di densità nel tema della v19 ed esponi un modo equivalente per leggerla nei componenti personalizzati. Il lavoro è completo quando gli sviluppatori possono recuperare la scala di densità del tema in Angular Material 19.

Scritto dal modello di indicizzazione a partire dal testo della issue.