[18.11] Fix session ID dropdown never appearing during active agentic chat

What does this MR do and why?

Backport of !231708 (merged) — Fix session ID dropdown never appearing during active agentic chat.

content_container.vue's showSessionId computed property checked activeTab.props?.mode === 'active', but activeTabObject is constructed 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.

Cherry-pick: Clean cherry-pick from merge commit d32ef925, no conflicts.

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