This is how it looks by default: JSON body has the collapse ability while headers do not.
I have checked the plugin's docs but don't see any acceptable solution.
Does not look like it's possible right now.
Here is the ticket that asks for the ability to collapse response headers:
https://youtrack.jetbrains.com/issue/IDEA-239338 -- watch it (star/vote/comment) to get notified with any progress.