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);

Comece em HttpServletSseServerTransportProvider, no handler GET que estabelece uma nova conexão SSE e envia o evento de endpoint inicial. Rastreie como o context path da request e o message endpoint estão disponíveis nesse ponto e, em seguida, verifique se o endpoint anunciado usa o context path para clientes atrás de um reverse proxy. O trabalho estará concluído quando a URL do endpoint for derivada da request em vez da baseUrl imutável.