fix(duo_chat_message): added explicit z-index

mentioned in commit 3c7d0e02

mentioned in merge request gitlab!200542 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1945 (merged)

mentioned in merge request gitlab!201287 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1947 (merged)

mentioned in merge request gitlab!201396 (closed)