fix: Hide Duo Agent Platform panel when feature flags are disabled

Description

1. Respect instance version instead of assuming enabled by default which is only true for 18.2 and later instances.
2. Respect feature flag rollout (e.g. enabled by default) status of feature flags for when we delete feature flags from the codebase.

Related Issues

Resolves [VSC] Hide Agentic Platform settings if feature... (#2050 - closed).

How has this been tested?

• If src/browser or src/common has been modified, please consider interoperability with the Web IDE. See Running the Extension in WebIDE.
• Consider an end-to-end test for significant new features that aren't covered by integration tests.

1. Run GDK locally.

2. Authenticate http://gdk.test:3000 with GitLab Workflow extension development host.

3. Open the GDK/GiLab Duo test project (or any Duo namespace+enabled project).

4. Enable both duo_agentic_chat and duo_workflow feature flags (default).

   # Check if they're already enabled:
   Feature.enabled?(:duo_agentic_chat)
   Feature.enabled?(:duo_workflow)
   
   # Enable both:
   Feature.enable(:duo_agentic_chat)
   Feature.enable(:duo_workflow)

5. Confirm you see the Agentic Tabs side panel when you authenticate with your GDK instance.

6. Disable duo_agentic_chat in the Rails console:

   Feature.disable(:duo_agentic_chat)

7. Run >Developer: Reload Webviews

   

8. Disable duo_workflow feature flag in Rails console:

   Feature.disable(:duo_workflow)

9. Run >Developer: Reload Webviews

10. Observe the agent platform side panel is no longer available after restarting the extension development host with the extension setting enabled:

    

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes

Closes #2050 (closed)