fix(DuoChat): move AI disclaimer above input

Jannik Lehmannrequested to merge
jnnkl-web-chat-disclaimer into main

What does this MR do?

Part of solving: https://gitlab.com/gitlab-org/gitlab/-/issues/576024

Move AI disclaimer from persistent sidebar to appear above the input box only when assistant messages exist.

before after
Screenshot_2025-11-13_at_14.01.36 2025-11-13_14-32-12
Edited by Jannik Lehmann

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