bug(material/core): Theme configuration checks should allow extra slots in color palettes

### 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 [Material Theme Builder](https://material-foundation.github.io/material-theme-builder/) generates slots 5 and 15 in the color palettes.

These should be allowed to be included in the color palettes even if they are not used by Angular Material.

However when I try to include those slots in a theme for Angular Material, the sass complier errors with "Expected $config.primary to be a valid M3 palette". The error is generated by https://github.com/angular/components/blob/c4337d635e3c87b029136c7eaa707df9f2a07163/src/material/core/theming/_config-validation.scss#L38 and https://github.com/angular/components/blob/c4337d635e3c87b029136c7eaa707df9f2a07163/src/material/core/theming/_config-validation.scss#L44

The validation checks should only fail on required (i.e. used by Angular Material) slots.

### Reproduction

StackBlitz link:
Steps to reproduce:
1. Generate a theme using `ng generate @angular/material:theme-color`
2. In any of the color palettes, add an extra value to the map, e.g. `5: #1e0037`.
3. `ng build`

### Expected Behavior

The build completes without error.

### Actual Behavior

The build failed with error.

```
✘ [ERROR] (Expected $config.primary to be a valid M3 palette. Got: (0: #000000, 5: #1e0037, ... )))
51 │ definition.define-colors($color));
│ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
╵
node_modules/@angular/material/core/tokens/_m3-system.scss 51:13 theme()
```

### Environment

- Angular: 19.2.5
- CDK/Material: 19.2.7

从 src/material/core/theming/_config-validation.scss 中第 38 行和第 44 行附近的检查开始，然后将 slot 5 添加到使用 ng generate @angular/material:theme-color 生成的调色板中并运行 ng build，以重现该失败。完成标准是：包含额外 slot 的生成调色板能够在没有有效 M3 调色板错误的情况下完成构建。

由索引模型根据 Issue 内容生成。