fixes [BUG: DropdownButtonFormField input decorator focus/hover is not clipped
](https://github.com/flutter/flutter/issues/131282)

### Description

This fixes an issue where `DropdownButtonFormField`'s input border isn't used for clipping with `InkWell`.

### Before
![Screenshot 2023-07-28 at 17 06 20](https://github.com/flutter/flutter/assets/48603081/8fe1ee1f-5cea-4297-b4f6-e672d74bb583)

### After
![Screenshot 2023-07-28 at 17 06 38](https://github.com/flutter/flutter/assets/48603081/a0091459-67dc-45ca-96b1-95d7093d475f)