Should McpTransportContextExtractor contain default methods for protocol version, lastEventId and sessionId

定位 McpTransportContextExtractor，并结合 McpTransportContext 的使用情况检查其现有实现。检查协议版本、会话 ID 和最后事件 ID 的默认访问器是否符合当前 API 和协议概念；完成的标准是该接口公开一致且兼容的 API。

I am defining my McpTransportContextExtractor implementation as:

@Internal
@Singleton
class DefaultMcpTransportContextExtractor implements McpTransportContextExtractor<HttpRequest<?>> {
    public static final String HTTP_HEADER_MCP_PROTOCOL_VERSION = "MCP-Protocol-Version";
    public static final String DEFAULT_PROTOCOL_VERSION = "2025-03-26";
    public static final String HTTP_HEADER_MCP_SESSION_ID = "Mcp-Session-Id";
    public static final String HTTP_HEADER_DEFAULT_LAST_EVENT_ID = "Last-Event-ID";

    @Override
    public McpTransportContext extract(HttpRequest<?> request, McpTransportContext transportContext) {
        transportContext.put(HTTP_HEADER_MCP_PROTOCOL_VERSION,
            request.getHeaders().get(HTTP_HEADER_MCP_PROTOCOL_VERSION, String.class)
                .orElse(DEFAULT_PROTOCOL_VERSION));
        request.getHeaders().get(HTTP_HEADER_MCP_SESSION_ID, String.class)
            .ifPresent(v -> transportContext.put(HTTP_HEADER_MCP_SESSION_ID, v));
        request.getHeaders().get(HTTP_HEADER_DEFAULT_LAST_EVENT_ID, String.class)
            .ifPresent(v -> transportContext.put(HTTP_HEADER_DEFAULT_LAST_EVENT_ID, v));
        return transportContext;
    }
}

But given that the specification defines session id, last event and protocol version. I think it would be good if the McpTransportContextExtractor contained default methods:

default String getSessionId() {
   return null;
}

default String getLastEventId() {
   return null;
}

default String getProtocolVersion() {
   return "2025-03-26";
}

I can submit a PR if you think it is a good idea. With this change the TransportContext will have a clear API for those spec defined concepts.