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);

從 HttpServletSseServerTransportProvider 開始，找到用來建立新 SSE 連線並傳送初始 endpoint 事件的 GET handler。追蹤 request context path 和 message endpoint 在那裡如何可用，然後驗證對於位於反向代理後的用戶端，公告的 endpoint 使用了 context path。完成的標準是 endpoint URL 從 request 推導，而不是從不可變的 baseUrl 推導。