When a QUIC session's MaxIdleTimeout expires, the underlying *quic.Conn dies
but the dead quicSession stays in the sessions map. The next Dial() finds
it and calls session.GetConn() which calls OpenStreamSync(context.Background())
against the dead connection — blocking indefinitely.
Add IsClosed() to both quicSession types (using the session context's Done()
channel) and add the liveness guard in both Dial() methods, matching the
existing pattern in KCP and MASQUE dialers.
Fixesgo-gost/gost#225
ginuerzh