Use explicit UTF-8 charset in StdioServerTransportProvider (#826)

Also add a test simulating a different locale in an isolated process.

Resolves #295

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

@@ -200,7 +200,7 @@ public class StdioServerTransportProvider implements McpServerTransportProvider
 					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();