fix: ensure Stdio transport threads are daemons and closed properly (#759)

- Use Schedulers.newSingle with daemon=true for StdioClientTransport and StdioServerTransportProvider.
- Explicitly close process streams in StdioClientTransport#closeGracefully to unblock reading threads.
- Removes unused Executors imports.

This prevents Stdio transport threads from lingering and blocking JVM exit.

Closes #759

Author: noxymon

mcp-core/src/main/java/io/modelcontextprotocol/client/transport/StdioClientTransport.java

@@ -11,7 +11,6 @@
 import java.time.Duration;
 import java.util.ArrayList;
 import java.util.List;
-import java.util.concurrent.Executors;
 import java.util.function.Consumer;
 import java.util.function.Function;
 
@@ -88,9 +87,9 @@ public StdioClientTransport(ServerParameters params, McpJsonMapper jsonMapper) {
 		this.errorSink = Sinks.many().unicast().onBackpressureBuffer();
 
 		// Start threads
-		this.inboundScheduler = Schedulers.fromExecutorService(Executors.newSingleThreadExecutor(), "inbound");
-		this.outboundScheduler = Schedulers.fromExecutorService(Executors.newSingleThreadExecutor(), "outbound");
-		this.errorScheduler = Schedulers.fromExecutorService(Executors.newSingleThreadExecutor(), "error");
+		this.inboundScheduler = Schedulers.newSingle("inbound", true);
+		this.outboundScheduler = Schedulers.newSingle("outbound", true);
+		this.errorScheduler = Schedulers.newSingle("error", true);
 	}
 
 	/**
@@ -348,6 +347,14 @@ public Mono<Void> closeGracefully() {
 		})).then(Mono.defer(() -> {
 			logger.debug("Sending TERM to process");
 			if (this.process != null) {
+				try {
+					this.process.getInputStream().close();
+					this.process.getErrorStream().close();
+					this.process.getOutputStream().close();
+				}
+				catch (IOException e) {
+					logger.warn("Failed to close process streams: {}", e.getMessage());
+				}
 				this.process.destroy();
 				return Mono.fromFuture(process.onExit());
 			}

mcp-core/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java

@@ -11,7 +11,6 @@
 import java.io.OutputStream;
 import java.nio.charset.StandardCharsets;
 import java.util.List;
-import java.util.concurrent.Executors;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Function;
 
@@ -151,10 +150,8 @@ public StdioMcpSessionTransport() {
 			this.outboundSink = Sinks.many().unicast().onBackpressureBuffer();
 
 			// Use bounded schedulers for better resource management
-			this.inboundScheduler = Schedulers.fromExecutorService(Executors.newSingleThreadExecutor(),
-					"stdio-inbound");
-			this.outboundScheduler = Schedulers.fromExecutorService(Executors.newSingleThreadExecutor(),
-					"stdio-outbound");
+			this.inboundScheduler = Schedulers.newSingle("stdio-inbound", true);
+			this.outboundScheduler = Schedulers.newSingle("stdio-outbound", true);
 		}
 
 		@Override