actionWidget: fix submenu group label rendered as item description (#306327)

The submenu header (e.g. 'Thinking effort') was incorrectly shown as the
description of the first action item instead of as a proper section header.

Regression from f6218ecb334 which replaced ActionListItemKind.Header items
with inline description on the first child action.

Restore proper Header rendering for SubmenuAction groups that have a label.
For the sessions workspace picker, move the provider label to the first
child action's tooltip so it renders as a description instead of a header.

Fixes #306250

Co-authored-by: Copilot <[email protected]>