close

Release the session's underlying network transport (Ktor HTTP client thread pool + connection pool). Idempotent. Once closed, the session cannot perform further operations — create a new one via SilentShard.ECDSA.createDuoSession(...) / EdDSA.createDuoSession(...) if needed. Default is a no-op so custom test stubs don't have to implement it; the SDK-provided session releases the HTTP client.