Tears down the conversation/session resources.
Configures the GraphQL provider with the guest token, boots the runtime, opens a conversation.
Persists the user's message, runs the pinned agent, returns its reply.
OptionalonProgress: WidgetProgressCallbackSwaps in a refreshed token (the next provider call uses it; conversation is preserved).
Reuses ConversationsRuntime + the GraphQL provider against MJAPI with the guest token.