Stream the vendor AI assistant's response as server-sent events (SSE). The endpoint emits
metadata, incremental content chunks, and a final summary event. Use this for real-time
chat experiences; the non-streaming endpoint is available at /vendors/chatbot/ask.

Event flow: metadata → tool_progress→ display_action? → content → actions? → final_metadata → done

Event semantics:

• metadata: Includes session_id, title, persona, model, and features.
• tool_progress: Emitted when the AI invokes a tool. Includes a title field
  with a human-readable label (e.g. "Starting your skin scan", "Thinking").
• display_action: A client-rendered card requested by the assistant (e.g. display_ticket_confirmation).
• NOTE: there is no products event. The assistant does not recommend catalogue products; a
  product question is answered with guidance plus a SKIN_SCAN action.
• content: A stream of assistant text chunks to render incrementally.
• actions: Emitted when the assistant triggers UI actions (e.g. SKIN_SCAN, BOOK_CONSULTATION).
  Includes actions array of action identifiers and action_links array of structured CTAs.
• final_metadata: Includes the saved AI message ID and mirrors persona fields for
  completeness once the response is finalized.
• error: Emitted on failure, then followed by done. When the vendor gates the chat
  (Tulip orders enabled) and the customer's email is not verified, the error carries
  code: "verification_required" — the client should run the POST /widget/chat/request-code
  → verify-code flow and retry with the resulting verification_token.
• done: Terminating event that signals stream completion.