WebBrokerAPI: Fix stale WebBrokerApi route handlers after xDS remove/update

Summary

AddWebBrokerApiBinding in event-gateway/gateway-runtime/internal/runtime/runtime.go
registers receivers on r.wsMux but never records the route path in bindingPaths.
As a result, RemoveWebBrokerApiBinding cannot remove the handler from the mux and
leaves stale WebSocket routes active after the binding is removed via xDS.

Steps to reproduce

1. Start the event gateway in xDS (control plane) mode.
2. Add a WebBrokerApi binding via xDS — a handler is registered on r.wsMux.
3. Delete or update the binding via xDS — RemoveWebBrokerApiBinding is called.
4. The route remains active on r.wsMux because the path was never stored in
   bindingPaths and r.websubMux.Remove is called instead of a removal on r.wsMux.

Required changes

• Record each registered WebBrokerApi path in bindingPaths[wbb.Name] inside
  AddWebBrokerApiBinding.
• Switch r.wsMux from *http.ServeMux to a removable mux implementation
  (e.g., the existing DynamicMux type, same as r.websubMux) so routes can be
  deregistered.
• Update RemoveWebBrokerApiBinding to look up paths in bindingPaths, call the
  removal API on r.wsMux, and delete the bindingPaths entry.

Related

• PR: https://github.com/wso2/api-platform/pull/1954
• Review comment: https://github.com/wso2/api-platform/pull/1954#discussion_r3239562652
• Related work: https://github.com/wso2/api-platform/pull/1944

Requested by

@senthuran16