chore(duo-agent-platform-v2): add feature toggle button

Description

Introduces a feature toggle button to switch between the classic and new Duo Agent Platform UI designs. Users can now easily toggle between the two UI versions instead of having two views available at the same time.

Added gitlab.featureFlags.duoAgentPlatformNext feature flag with three states:

• true - Show new UI with toggle button
• false - Show old UI with toggle button
• null (unset) - Show old UI without toggle button

Before	After

Related Issues

gitlab-org/editor-extensions/gitlab-lsp#1656 (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.

Update settings.json with and test three different states of gitlab.featureFlags.duoAgentPlatformNext:

"gitlab.featureFlags.duoAgentPlatformNext": true	"gitlab.featureFlags.duoAgentPlatformNext": false	"gitlab.featureFlags.duoAgentPlatformNext": null or unset
Shows new UI and button to switch to class design is available	Shows old UI and button to switch to new design is available	Shows old UI and no button to switch

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