chore: Add a developer facing FF for flow merge

Description

This change adds a new feature flag called "UseDuoChatUiForFlow" to the codebase. This will be used for progressively migrating over the flow tab to use the Duo Chat UI.

&18217 (comment 2856765333)

Related Issues

&18217 (comment 2856765333)

Screen_Recording_2025-10-30_at_15.31.00

How has this been tested?

1. Pull this branch down for the VSE project
2. Pull gitlab-org/editor-extensions/gitlab-lsp!2440 (merged) LSP
3. Execute npm run duo:agentic-chat
4. Start the development host debugger
5. Turn the Feature flag on in your user settings (CMD + SHIFT + P => User settings json) "gitlab.featureFlags.useDuoChatUiForFlow": true,

• 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