bug(form-field): label-offset-x is incorrectly computed when there is a prefix

### 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

Inside `_mdc-text-field-structure-overrides.scss`
https://github.com/angular/components/blob/7bb0a82a434c40d01a0d1aa35bcddaa3f725eccb/src/material/form-field/_mdc-text-field-structure-overrides.scss#L97
We can see that the variable `--mat-mdc-form-field-label-offset-x` is hardcoded to -16px (I guess it's coming from the fact that out of the box, form-field have a left-padding of 16px).

But for outlined form field, the left-padding could get bigger in some cases when using a large value for the border-radius.

This is because the left-padding is defined with `max(16px, var(--mdc-outlined-text-field-container-shape) + 4px)`

So if I want my outlined form-fields to be rounded like a pill, I have to override the border-radius with this
```
@include mat.form-field-overrides((
outlined-container-shape: 28px,
))
```

Which will set the left-padding to 32px and not 16px.

At this point everything looks good, and the label is correctly placed (when empty, it's aligned with the input value/placeholder)

But things start to break when you add a prefix as it will `translateX` the label to make room for the prefix.

This transform is based on the hardcoded value of `--mat-mdc-form-field-label-offset-x: -16px;`, so it works great for out-of-the-box form-field, but doesn't work for my pill form-field.

Because the padding is set to 32px, the `--mat-mdc-form-field-label-offset-x` should be -32px and not -16px.

So I end up with this:

As you can see, the empty pill form-field's label isn't aligned anymore with where the input's value is.

I know this can be fixed by overriding the `--mat-mdc-form-field-label-offset-x` myself by using `::ng-deep` to override the hardcoded value, but it's not something trivial to understand and fix, so I'd rather see a fix in the Angular Material repository.

### Reproduction

https://stackblitz.com/edit/tk7fhpq7-w7dz4vdj?file=src%2Fexample%2Fform-field.scss

### Expected Behavior

The label should be correctly placed when using a large border-radius value for outlined form-field, like the input value is.

I believe the solution would be to change this line of code:
```diff
- --mat-mdc-form-field-label-offset-x: -16px;
+ --mat-mdc-form-field-label-offset-x: max(16px, var(--mdc-outlined-text-field-container-shape, var(--mat-sys-corner-extra-small)) + 4px) * -1;
```

By the way, it would be nice to be able to control the "minimum" left & right padding to something else than the hardcoded 16px too 😅

### Actual Behavior

The label is not correctly aligned.

### Environment

- Angular: 20.0.4
- CDK/Material: 20.0.4
- Browser(s): Chrome 137
- Operating System: macOS

issue に記載されている label-offset-x 宣言がある src/material/form-field/_mdc-text-field-structure-overrides.scss から始め、次に outlined field、large container shape、prefix を使用したリンク先の StackBlitz reproduction を使います。空のフィールドについて、label と input value の位置合わせを確認します。その構成で label が正しく位置合わせされたままであれば完了です。

索引モデルが issue の本文から書いたものです。