mcp: add Session.ID (#67)

Support retrieving the session ID from client and server sessions.

Fixes #65.

mcp/server.go

@@ -505,6 +505,7 @@ func (ss *ServerSession) NotifyProgress(ctx context.Context, params *ProgressNot
 type ServerSession struct {
 	server           *Server
 	conn             *jsonrpc2.Connection
+	mcpConn          Connection
 	mu               sync.Mutex
 	logLevel         LoggingLevel
 	initializeParams *InitializeParams
@@ -512,6 +513,17 @@ type ServerSession struct {
 	keepaliveCancel  context.CancelFunc
 }
 
+func (ss *ServerSession) setConn(c Connection) {
+	ss.mcpConn = c
+}
+
+func (ss *ServerSession) ID() string {
+	if ss.mcpConn == nil {
+		return ""
+	}
+	return ss.mcpConn.SessionID()
+}
+
 // Ping pings the client.
 func (ss *ServerSession) Ping(ctx context.Context, params *PingParams) error {
 	_, err := handleSend[*emptyResult](ctx, ss, methodPing, params)