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

Bắt đầu với src/material/core/theming/_config-validation.scss tại các bước kiểm tra quanh dòng 38 và 44, sau đó tái hiện lỗi bằng cách thêm slot 5 vào một palette được tạo bằng ng generate @angular/material:theme-color và chạy ng build. Hoàn thành khi palette được tạo có các slot bổ sung được build mà không gặp lỗi palette M3 hợp lệ.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.