Fix session ID dropdown never appearing during active agentic chat

What does this MR do and why?

Fixes the session ID copy dropdown never appearing during active agentic chat.

content_container.vue's showSessionId computed property checked activeTab.props?.mode === 'active', but ai_panel.vue constructs activeTabObject as { title: this.currentTabTitle } — no props field. The condition was permanently false, so the "Copy Chat Session ID" ellipsis dropdown never rendered.

Fix: Replace the broken activeTab.props?.mode guard with a direct $route?.name === AGENTIC_CHAT_SHOW_ROUTE check. This is reactive (navigating away hides the dropdown, navigating back shows it), requires no changes to ai_panel.vue or the state manager, and follows the same pattern used by isShowingAgentSession in ai_panel.vue.

References

• The activeTab.props?.mode pattern was never wired up — activeTabObject has never included a props key.
• content_container.vue already watches $route.path, so $route access is established.

Screenshots or screen recordings

How to set up and validate locally

1. Open any project page (e.g., http://gdk.test:3000/gitlab-duo/test)
2. Click "Active GitLab Duo Chat" to open the AI panel
3. Send a message and wait for the agentic workflow to start streaming
4. Before fix: No ellipsis button appears in the chat header
5. After fix: An ellipsis (⋮) "More options" button appears in the header. Clicking it shows "Copy Chat Session ID: <id>"
6. Navigate to history tab → dropdown disappears
7. Navigate back to active chat → dropdown reappears

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