Improve how text header values are rendered
Currently it looks a bit weird how text values in record headers are shown. This is probably because we have support for other presentation modes (i. e. JSON). If necessary we could add some "special handling" for the common case where header values are just plain strings:

I think for anything that doesn't contain a line-break and is below ~50 characters, the "expand" button should be hidden.
As for the actual rendering (when it is expanded): Yes it looks a bit strange, but I'm not sure what to change about it. 😂 🤷♂️
Maybe we could also just render the headers using the json viewer.
cc @rikimaru0345 just added this to the message viewer tasklist
Done, will be deployed with next release