Add test for UTF-8 encoded messages in STDIO transport

Verifies that multi-byte UTF-8 characters (Korean, emoji, accented
Latin, Japanese) are correctly preserved through the STDIO inbound
processing pipeline.

Author: xxxxxxjun

mcp-test/src/test/java/io/modelcontextprotocol/server/transport/StdioServerTransportProviderTests.java

@@ -135,6 +135,49 @@ void shouldHandleIncomingMessages() throws Exception {
 		}).verifyComplete();
 	}
 
+	@Test
+	@SuppressWarnings("unchecked")
+	void shouldHandleUtf8EncodedMessages() throws Exception {
+
+		String utf8Content = "한글 테스트 🎉 café 日本語";
+		String jsonMessage = "{\"jsonrpc\":\"2.0\",\"method\":\"test\"," + "\"params\":{\"message\":\"" + utf8Content
+				+ "\"},\"id\":1}\n";
+		InputStream stream = new ByteArrayInputStream(jsonMessage.getBytes(StandardCharsets.UTF_8));
+
+		transportProvider = new StdioServerTransportProvider(McpJsonDefaults.getMapper(), stream, System.out);
+
+		AtomicReference<McpSchema.JSONRPCMessage> capturedMessage = new AtomicReference<>();
+		CountDownLatch messageLatch = new CountDownLatch(1);
+
+		McpServerSession.Factory realSessionFactory = transport -> {
+			McpServerSession session = mock(McpServerSession.class);
+			when(session.handle(any())).thenAnswer(invocation -> {
+				capturedMessage.set(invocation.getArgument(0));
+				messageLatch.countDown();
+				return Mono.empty();
+			});
+			when(session.closeGracefully()).thenReturn(Mono.empty());
+			return session;
+		};
+
+		transportProvider.setSessionFactory(realSessionFactory);
+
+		StepVerifier.create(Mono.fromCallable(() -> messageLatch.await(100, TimeUnit.SECONDS)).flatMap(success -> {
+			if (!success) {
+				return Mono.error(new AssertionError("Timeout waiting for message processing"));
+			}
+			return Mono.just(capturedMessage.get());
+		})).assertNext(message -> {
+			assertThat(message).isNotNull();
+			assertThat(message).isInstanceOf(McpSchema.JSONRPCRequest.class);
+			McpSchema.JSONRPCRequest request = (McpSchema.JSONRPCRequest) message;
+			assertThat(request.method()).isEqualTo("test");
+			assertThat(request.id()).isEqualTo(1);
+			Map<String, Object> params = (Map<String, Object>) request.params();
+			assertThat(params).containsEntry("message", utf8Content);
+		}).verifyComplete();
+	}
+
 	@Test
 	void shouldNotifyClients() {
 		// Set session factory