Add per-visitor session management for the WebChat Web/API access mode:
list a visitor's conversation threads, fetch a thread's messages, and
delete a thread.
Authorization: visitorId is a client-asserted request param, so it cannot
be trusted on its own — deriving conversationId from it and then checking
ownership against it is tautological (any caller passes). Instead, /stream
issues a per-visitor token = HMAC-SHA256(jwtSecret, channelId:visitorId),
returned in the meta event; the management endpoints require it back via
the X-MC-Visitor-Token header and verify it in constant time. The signing
secret is server-only (unlike the public channel API key) and the channelId
in the payload makes tokens non-portable across channels.
Includes regression tests for token issuance/verification semantics
(forged visitorId rejected, cross-visitor and cross-channel tokens rejected,
tampered tokens rejected).
77b6baeccc