Credit popup vertical re-centering

### What happened?

Credit display popup should vertically re-center when content changes. Currently popup size is only centered the first time it is opened or when user changes viewport size. Follow up from https://github.com/CesiumGS/cesium/pull/12913#issuecomment-3356453416

### Reproduction steps

1. Load sandcastle with imagery layer using per-tile/viewport dynamic attribution, eg. [sandcastle](https://sandcastle.cesium.com/#c=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) loads Google Maps Satellite which has per-tile/viewport attribution
2. Click "Data attribution" link in the bottom left of the viewer, the credits list in the attribution should be short and the popup should be vertically centered
3. Close the popup, zoom in (height around 5000 meters will definitely reproduce it), check the data attribution popup again, if the credit list content is longer, you will see the popup has not re-centered and the content runs off the screen

### Sandcastle example

_No response_

### Environment

Browser:
CesiumJS Version:
Operating System: