bug(mat-date-range-input): formGroup error doesn't switch form field error state

### 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

Hello,

https://material.angular.io/components/datepicker/overview#date-range-input-forms-integration
The mat date range input documentation states:

> **Date range input forms integration**
>
> The mat-date-range-input component can be used together with the FormGroup directive from @angular/forms to group the start and end values together and to validate them as a group.

So I'm expecting the form group, grouping start and end date, to update the "global range input" state, but this is not the case.

My use case is to add a customValidator (to limit the number of days between start and end dates) on the formGroup. When the form is in error, I was expecting the form field to be red and to display mat-error, but nothing shows when the form is invalid.

- Is it a bug?
- Is it the normal behavior, and a documentation ambiguity? It would be a nice new feature in this case.

I've found a workaround using ErrorStateMatcher. But it feels like it's supposed to be included in mat date range picker as both inputs are inside, within the formGroup
https://material.angular.io/components/input/overview#changing-when-error-messages-are-shown

### Reproduction

Have mat date range picker with a form group validator => only start and end controls are updating displayed form field validity

Reproduction link: https://stackblitz.com/edit/stackblitz-starters-xhx4k1?file=src%2Fmain.ts

### Expected Behavior

When formGroup is in error, form field state should be in error and display mat-error

### Actual Behavior

When formGroup is in error, form field state is valid and no mat-error is displayed

### Environment

- Angular: 18
- CDK/Material: 18

リンクされている StackBlitz の再現を開き、まず date-range input forms の統合ドキュメントを読んでください。form-group validator が date-range フィールドのエラー状態にどのような影響を与えるかを追跡してください。無効なグループがフィールドをエラーとしてマークし、mat-error を表示し、有効なグループは変更されない状態になれば完了です。

索引モデルが issue の本文から書いたものです。