fix(stdio): escape newlines in JSON messages per MCP spec

Resolves #21

spring-ai-mcp-core/src/main/java/org/springframework/ai/mcp/client/stdio/StdioServerTransport.java

@@ -28,6 +28,7 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+import reactor.core.Disposable;
 import reactor.core.publisher.Mono;
 import reactor.core.scheduler.Scheduler;
 import reactor.core.scheduler.Schedulers;
@@ -225,6 +226,9 @@ private void startOutboundProcessing() {
 				if (message != null) {
 					try {
 						String jsonMessage = objectMapper.writeValueAsString(message);
+						// Escape any embedded newlines in the JSON message as per MCP
+						// spec
+						jsonMessage = jsonMessage.replace("\r\n", "\\n").replace("\n", "\\n").replace("\r", "\\n");
 						this.process.getOutputStream().write(jsonMessage.getBytes(StandardCharsets.UTF_8));
 						this.process.getOutputStream().write("\n".getBytes(StandardCharsets.UTF_8));
 						this.process.getOutputStream().flush();

spring-ai-mcp-core/src/test/java/org/springframework/ai/mcp/spec/DefaultMcpSessionTests.java

@@ -31,7 +31,6 @@
 import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
-import org.springframework.ai.mcp.client.McpAsyncClient;
 import org.springframework.ai.mcp.spec.McpSchema.JSONRPCNotification;
 import org.springframework.ai.mcp.spec.McpSchema.JSONRPCRequest;