Tears down the conversation/session resources.
Initializes the transport with the freshly-minted guest session.
Sends a user message and resolves with the agent's reply. The pinned agent id from the session is ALWAYS passed as explicitAgentId (D5) — the transport never lets the caller pick an arbitrary agent.
OptionalonProgress: WidgetProgressCallbackUpdates the held token after a refresh (no reconnection of the conversation).
Abstraction over "start a conversation and exchange turns with the pinned agent." One transport instance per mounted widget session.