Specify UTF-8 charset explicitly in STDIO transport InputStreamReader

The MCP specification requires UTF-8 encoding for STDIO transport
messages, but some InputStreamReader instances were created without
an explicit charset, falling back to the JRE default encoding which
may not be UTF-8 on all platforms.

This change adds StandardCharsets.UTF_8 to all InputStreamReader
constructors in StdioClientTransport (stdin and stderr readers)
and StdioServerTransportProvider (stdin reader) for consistency
with the output streams that already specify UTF-8.

Resolves #295

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();