feat: enable agentic chat placeholder

Description

The MR sets up a placeholder for the agentic chat UI. The availability of the agentic chat UI is controlled with the newly-introduced (in this MR) setting gitlab.duoAgenticChat.enabled

With setting ON	With setting OFF (default)

Related Issues

Related to #1922 (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.

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