This PR fixes `SliverCrossAxisGroup` so that once we calculate the total `scrollExtent` of the `SliverCrossAxisGroup`, we ensure that any child sliver is not painted outside of the "scroll extent" of the sliver group.

https://github.com/flutter/flutter/assets/2004742/670dc6f9-a3c6-4bcc-85d3-576cf7f33c6a

https://github.com/flutter/flutter/assets/2004742/0c9cd951-c133-4a8b-9e5e-89d0a69a4f59

Fixes #126958.
Fixes #126957.