[Dev UI] Markdown/plain text toggle doesn't do anything for model responses in trace

[apascal07]

Describe the bug
Markdown/plain text toggle doesn't do anything for model responses in trace, it just kinda adds weird spaces around the chrome.

To Reproduce
Go to a trace with a model response that is a tool request, and toggle between markdown and plain text.

Expected behavior
No toggle since it doesn't do anything, like for tool responses.

Screenshots

[MichaelDoyle]

Context

• Model messages can include text, thoughts, etc interleaved with tool requests. So there is a valid need for the toggle. Could we hide it, if there are none of these? Yes - but it might be better to leave it for visual continuity.
• Adding weird spaces is not intended and should probably be addressed

@apascal07 you have an example trace we can use to debug?

[apascal07]

@MichaelDoyle Noticed it when testing sample in this PR: #4112