Make Delete Cell button reachable via arrow keys in notebook cell toolbar

[Copilot]

The notebook cell title toolbar rendered the main actions and the Delete Cell action as two separate ToolBar instances inside the same container. Since arrow-key navigation is scoped to a single ActionBar, keyboard users had to press Tab to move focus from the main toolbar actions onto the Delete Cell button, breaking the expected navigation pattern within a single toolbar group.

Description

• Single toolbar rendering (src/vs/workbench/contrib/notebook/browser/view/cellParts/cellToolbars.ts): Dropped the separate deleteToolbar/createDeleteToolbar helper and render the delete menu's actions in the existing WorkbenchToolBar, appending them after the title actions to preserve visual order.
• Change listeners: setupChangeListeners now subscribes to both titleMenu and deleteMenu change events and recomputes the combined action list, so contributions to either menu still update correctly. The dropdown-deferred-update path is preserved.
• Menus unchanged: MenuId.NotebookCellTitle and MenuId.NotebookCellDelete remain distinct, so extension contributions to either continue to work.

// Combine the main title actions and delete actions into a single toolbar
// so that arrow-key navigation reaches all items (including Delete Cell).
const primary = model.actions.primary.concat(model.deleteActions.primary);
const secondary = model.actions.secondary.concat(model.deleteActions.secondary);
toolbar.setActions(primary, secondary);