fix: use UTF-8 charset in StdioClientTransport stream readers

InputStreamReader defaults to the system charset, which can corrupt
MCP messages on non-UTF-8 JVMs. Use StandardCharsets.UTF_8 explicitly
in both startInboundProcessing and startErrorProcessing, consistent
with the outbound writer which already uses UTF-8.

Fixes #898

Signed-off-by: Gorre Surya <suryateja.g13@gmail.com>

Author: suryateja-g13

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 {