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);

从 HttpServletSseServerTransportProvider 开始，找到用于建立新 SSE 连接并发送初始 endpoint 事件的 GET handler。跟踪 request context path 和 message endpoint 在那里如何可用，然后验证对于位于反向代理后的客户端，公布的 endpoint 使用了 context path。完成的标准是 endpoint URL 从 request 推导，而不是从不可变的 baseUrl 推导。