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

首先，将 Angular Material v18 主题指南中的“reading the density scale”入口点与 v19 主题 API 进行比较。确定密度刻度在 v19 主题中的表示方式，并为自定义组件提供一种等效的读取方式。完成的标准是开发者可以在 Angular Material 19 中获取主题密度刻度。

由索引模型根据 Issue 内容生成。