fix(DuoChat): don't show cancel button on slash commands

mentioned in commit c9f16d17

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!607 (closed)

mentioned in merge request status-page!903

mentioned in merge request gitlab!157024 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!338 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!40 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3917 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!645 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!726 (closed)