feat: Update Agentic features guards

Description

We'll have a single flag that controls both chat and workflow in the two-tabbed interface. Standalone sidebar items won't be there.

This MR:

• Renamed configuration setting from duoAgenticChat to duoAgentPlatform
• Updated the temporary robot icon for the 2-tabbed interface to the one that is used by the Agentic Chat
• Updated the conditions under which different sidebar views show

Related Issues

Closes gitlab-org/editor-extensions/gitlab-lsp#1251 (closed)

Related to gitlab-org/editor-extensions/gitlab-lsp#1217 (closed)

How has this been tested?

1. Make sure to link related LS MR
2. Set the development feature flag "gitlab.featureFlags.agenticTabs": true in the extension settings.json (this is additional dev feature flag hidden from the user. We'll keep it as it will allow to roll-out Agent Platform tabs just toggling the value and on the other hand will allow to turn off agentic tabs and go back to separate sidebar items just in case it is needed) We're waiting for the chat history buttons to be merged before the flag can be set to true.)
3. Open extension settings and enable Agent Platform flag (it is enabled by default)
4. Agentic Platform (previous agentic chat icon) should be visible in the sidebar

4. Disable the flag and the sidebar item won't be a available
5. If you change the feature flag to "gitlab.featureFlags.agenticTabs": false in settings.json you should be able to see 2 sidebar items (separately Agentic Chat and Workflow when the User Setting is ON

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