The generic websocket listener now returns the handler map
with the resolved options attached in its metadata (under the
::options key), exposing the codecs and internal state for
testing and introspection purposes without affecting the
public api.
The http-handler of the notifications endpoint is now public,
and the websocket tests are extended to cover the wiring of
the negotiated payload format into the listener: json codecs
selected via Accept header or _fmt parameter, and the transit
default for unsignaled requests.
Closes#11409
AI-assisted-by: glm-5.3-flash
11eb60712a