Drop placeholder category from focus-view commands in generated containers

[Copilot]

When a view is moved out of its default container (e.g. dragging Outline from the Explorer to the Activity Bar), it lands in an auto-generated ViewContainer whose title is the internal placeholder "User View Container". ViewsService was using that title as the category for the view's Focus on … View action, so the Command Palette ended up showing entries like User View Container: Focus on Outline View.

Changes:

• viewsService.ts: when registering the focus action, pass undefined as the category if the host container is generated, so the Command Palette shows just Focus on Outline View. Behavior for views in regular containers (e.g. Explorer: Focus on Outline View) is unchanged.
• IViewDescriptorService: expose the previously private isGeneratedContainerId(id) so callers can identify auto-generated containers without reaching into internals.

disposables.add(this.registerFocusViewAction(
    viewDescriptor,
    this.viewDescriptorService.isGeneratedContainerId(container.id) ? undefined : container.title
));

This aligns with the existing intent already noted at the placeholder definition: // having a placeholder title - this should not be shown anywhere.