feat(responses-ws): extend channel support and share routing with HTTP

Allow Advanced Custom, Sub2API and New API channels for Responses
WebSocket when the per-channel toggle is on. Advanced Custom requires a
converter-free /v1/responses route, and editing the route's upstream
path, conversion or credential placement forces a reconnect.

Match token model limits the same way HTTP does, share channel selection
and used-channel tracking between the distributor and the WebSocket
session, record channel affinity on the first accepted upstream event,
parse the create envelope once, restore the default type for synthesized
upstream errors, fold proxy, request context and handshake status into
DoWssRequest, send the upstream close frame on policy close, drop the
unobservable policy defer lines and cover the control and late-error
branches that had no tests.