feat: change agentic platform webview availability logic

Description

This MR implement the vscode side of gitlab-org/editor-extensions/gitlab-lsp#1848 (closed).

It is a draft for now as it depends on feat: add agent_platform_ga_rollout check (gitlab-org/editor-extensions/gitlab-lsp!2713 - merged) to be merged.

Related Issues

Issue #1848 (closed)

How has this been tested?

1. Connect to the local gitlab instance (with gdk, something like http://gdk.test)
2. Enable ai_duo_agent_platform_ga_rollout_self_managed feature flag in rails console (run rails c command in gitlab project folder): Feature.enable("ai_duo_agent_platform_ga_rollout_self_managed"). If your gdk instance is configured to emulate SAAS, use ai_duo_agent_platform_ga_rollout feature flag instead
3. checkout this LS MR
4. run npm run watch -- --editor=vscode in LSP
5. in the LSP repository run npm run watch -- --editor=vscode command
6. in the vscode repository, run the extension
7. If the ai_duo_agent_platform_ga_rollout_self_managed flag is enabled, you should see the label adjusted:

FF is disabled	FF is enabled

⚠️ Does this MR include a Duo UI update? If so, please smoke test all webviews in a downstream project to ensure there are no visual regressions

• One of:
  • GitLab Workflow Extension for VS Code (setup instructions).
  • GitLab JetBrains Plugin (setup instructions).

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
• other non-user-facing changes