fix: resolve type inference issues in StdioClientTransport

- Add explicit Void type parameter to Mono.defer() call
- Chain .then() to Mono.delay() to ensure consistent Mono<Void> return type

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

Author: tzolov

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

@@ -346,14 +346,14 @@ public Mono<Void> closeGracefully() {
 		return Mono.fromRunnable(() -> {
 			isClosing = true;
 			logger.debug("Initiating graceful shutdown");
-		}).then(Mono.defer(() -> {
+		}).then(Mono.<Void>defer(() -> {
 			// First complete all sinks to stop accepting new messages
 			inboundSink.tryEmitComplete();
 			outboundSink.tryEmitComplete();
 			errorSink.tryEmitComplete();
 
 			// Give a short time for any pending messages to be processed
-			return Mono.delay(Duration.ofMillis(100));
+			return Mono.delay(Duration.ofMillis(100)).then();
 		})).then(Mono.defer(() -> {
 			logger.debug("Sending TERM to process");
 			if (this.process != null) {