Resolve message endpoint path using request context path

When HttpServletSseServerTransportProvider handling the GET request to establish a new SSE connection, the message endpoint is advertised using the immutable field "baseUrl" that's set in the constructor:

// Send initial endpoint event this.sendEvent(writer, ENDPOINT_EVENT_TYPE, this.baseUrl + this.messageEndpoint + "?sessionId=" + sessionId);

This will not work if clients use a different base URL to access this servlet (e.g.: using reverse proxies). The servlet implementation should allow the request context path to be used to produce the endpoint path appropriate for that request:

// Send initial endpoint event this.sendEvent(writer, ENDPOINT_EVENT_TYPE, request.getContextPath() + this.messageEndpoint + "?sessionId=" + sessionId);

Start in HttpServletSseServerTransportProvider, at the GET handler that establishes a new SSE connection and sends the initial endpoint event. Trace how the request context path and message endpoint are available there, then verify that the advertised endpoint uses the context path for clients behind a reverse proxy. Done means the endpoint URL is derived from the request rather than the immutable baseUrl.