Add editor part chevron to toggle the secondary side bar in the Agents window

[sandy081]

Implemented via #319987.

Feature

Adds a chevron toggle to the editor part title bar (right of the tabs) in the Agents window to quickly widen the editor by collapsing the secondary side bar (auxiliary bar).

Verification steps

1. Open the Agents window and open a file so the editor is shown inside the window (not as a modal) with the secondary side bar visible.
2. In the editor title bar (right of the tabs), click the chevron-right ("Push Editor Right") action.
   • ✅ The secondary side bar is hidden and the editor widens.
3. The same slot now shows a chevron-left ("Show Secondary Side Bar") action. Click it.
   • ✅ The secondary side bar is restored.
4. Verify the chevron direction flips correctly based on auxiliary bar visibility.
5. Verify the action is only shown for the Agents window editor (main editor group, grid mode) and not when the editor is modal.