Fix active nav state for AI features

What does this MR do and why?

Sub-items in the AI / Automate super sidebar section (Agents, Flows, Triggers, Sessions, MCP servers, Onboarding) never highlighted as active when their page was open.

References

N/A

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Ensure you have a group/project with GitLab Duo / AI features enabled so the AI (or Automate) section appears in the super sidebar.
2. Visit each AI page and confirm the corresponding sidebar item is highlighted as active:
   • /-/automate/agents
   • /-/automate/flows
   • /-/automate/triggers
   • /-/automate/agent-sessions
   • /-/automate/mcp-servers
   • /-/automate/onboarding
3. Repeat the checks at the group level (/groups/<group>/-/automate/...) for Agents, Flows, and MCP servers.
4. Validate with the :hide_unpinned_sidebar_items feature flag disabled (default):

   Feature.disable(:hide_unpinned_sidebar_items)

   Confirm each page above highlights the correct sidebar item.
5. Validate with the :hide_unpinned_sidebar_items feature flag enabled:

   Feature.enable(:hide_unpinned_sidebar_items)

   Confirm the active state still resolves correctly for each page (item highlighting and, where applicable, the section/flyout showing as active).

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