Fix potential encoding issue in STDIO transport

Explicitly specify StandardCharsets.UTF_8 in InputStreamReader
constructors for both server and client STDIO transports to ensure
consistent encoding regardless of the JRE default charset.

The MCP specification requires UTF-8 encoding for STDIO transport
messages. The outbound processing already uses UTF_8 explicitly, but
the inbound processing relied on the platform default charset, which
could cause encoding mismatches on non-UTF-8 systems.

Resolves #295

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: xxxxxxjun

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

@@ -180,7 +180,7 @@ public void awaitForExit() {
 	private void startErrorProcessing() {
 		this.errorScheduler.schedule(() -> {
 			try (BufferedReader processErrorReader = new BufferedReader(
-					new InputStreamReader(process.getErrorStream()))) {
+					new InputStreamReader(process.getErrorStream(), StandardCharsets.UTF_8))) {
 				String line;
 				while (!isClosing && (line = processErrorReader.readLine()) != null) {
 					try {
@@ -246,7 +246,8 @@ public Mono<Void> sendMessage(JSONRPCMessage message) {
 	 */
 	private void startInboundProcessing() {
 		this.inboundScheduler.schedule(() -> {
-			try (BufferedReader processReader = new BufferedReader(new InputStreamReader(process.getInputStream()))) {
+			try (BufferedReader processReader = new BufferedReader(
+					new InputStreamReader(process.getInputStream(), StandardCharsets.UTF_8))) {
 				String line;
 				while (!isClosing && (line = processReader.readLine()) != null) {
 					try {

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

@@ -200,7 +200,7 @@ private void startInboundProcessing() {
 					inboundReady.tryEmitValue(null);
 					BufferedReader reader = null;
 					try {
-						reader = new BufferedReader(new InputStreamReader(inputStream));
+						reader = new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8));
 						while (!isClosing.get()) {
 							try {
 								String line = reader.readLine();