Change Request: Preserve the original casing of selectors in `no-duplicate-keyframe-selectors` error messages

### Environment

ESLint version: 10.8.0
@eslint/css version: ^2.0.0
Node version: v22.18.0
npm version: v10.9.3
Operating System: darwin 24.6.0

### What problem do you want to solve?

- `no-duplicate-keyframe-selectors` correctly detects duplicate selectors regardless of casing, but its error messages display the selector in lowercase rather than as written.

```css
/* eslint css/no-duplicate-keyframe-selectors: "error" */

@keyframes test {
from {
opacity: 0;
}
to {
opacity: 1;
}
TO {
opacity: 2;
}
}
```

- The current error message is:
```
Unexpected duplicate selector 'to' found within keyframe block.
```

- The error location points to `TO`, but the message displays to.
- Preserving the reported selector’s original casing could make it easier to match the message to the code.
- The same plugin’s `prefer-logical-properties` rule preserves the original casing in the following examples: [StackBlitz reproduction](https://stackblitz.com/edit/vitejs-vite-18mmtr5a?file=package.json,test.css,eslint.config.js,.vscode%2Fsettings.json,integrations.vscode.userData.settingsRaw).
```css
/* eslint css/prefer-logical-properties: "error" */

.example {
MARGIN-LEFT: 16PX;
text-align: RIGHT;
}
```
```
Expected logical property 'margin-inline-start' instead of 'MARGIN-LEFT'.
Expected logical value 'end' instead of 'RIGHT'.
```

### What do you think is the correct solution?

- I’d suggest keeping the value used for duplicate detection separate from the value displayed in the error message, so the message preserves the reported selector’s original casing.
- The expected message for the example above would be:
```
Unexpected duplicate selector 'TO' found within keyframe block.
```

### Participation

- [x] I am willing to submit a pull request for this change.

### AI acknowledgment

- [ ] I did not use AI to generate this issue report.
- [x] (If the above is not checked) I have reviewed the AI-generated content before submitting.

### Additional comments

> Disclosure: I'm a participant of [open source contribution program OSSCA](https://github.com/eslint-ossca)