inlineChat: make status placeholder registration a singleton (#312975)

* inlineChat: make status placeholder registration a singleton

The actionViewItemService.register() call for the StatusPlaceholder
was done per InlineChatZoneWidget instance. Since the service throws
on duplicate (menuId, commandId) registrations, having two zone widgets
(e.g. in side-by-side editors) would crash.

Fix: single process-lifetime registration that dispatches to the correct
widget instance via DOM containment at render time.

Fixes #312792

* defer DOM lookup to rAF since render() runs before DOM attachment