Restore Duration import removed by PR #1

Author: Mark Pollack

acp-core/src/main/java/com/agentclientprotocol/sdk/client/transport/StdioAcpClientTransport.java

@@ -8,6 +8,7 @@
 import java.io.IOException;
 import java.io.InputStreamReader;
 import java.nio.charset.StandardCharsets;
+import java.time.Duration;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.Executors;