Fix layout jank when scrolling back up to post body

[hjiangsu]

Pull Request Description

This PR fixes an issue where scrolling up on the post page often times results in a layout shift. This issue comes up due to the use of expandable in the post body. Whenever the post body is re-introduced into the widget tree (lazy loading of the sliver list), the expandable animation replays and causes the whole layout to shift downwards to accomodate for the body expansion.

This is an example of the layout shift that occurs in the post body (issue occurs at ~7 seconds in):

ScreenRecording_01-17-2025.11-20-08_1.mp4

The fix here is to apply the AutomaticKeepAliveClientMixin to _PostSubviewState in order to stop the expansion animation from happening:

ScreenRecording_01-17-2025.11-39-48_1.mp4

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?

[micahmo]

Very nice!