Do not steal focus from quick picks during startup

Guard focus calls in NewChatViewPane and SessionsWelcomeOverlay
by checking IQuickInputService.currentQuickInput before focusing.
This prevents the sessions welcome page from closing an already
open quick pick (e.g. Open Recent) and stealing focus to the
chat input box.

Fixes microsoft/vscode#294505

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