Fix UTF-8 encoding for non-ASCII tool names in HTTP client transports

Both HttpClientSseClientTransport and HttpClientStreamableHttpTransport
set Content-Type to 'application/json' without specifying the charset.
While Java's BodyPublishers.ofString() uses UTF-8 by default, the
missing charset in the header can cause the server to interpret the
request body using a different encoding (e.g., ISO-8859-1), corrupting
non-ASCII characters such as Chinese tool names.

Explicitly set Content-Type to 'application/json; charset=utf-8' in
POST requests on both client transports.

Fixes #260

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rameshreddy-adutla

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

@@ -456,7 +456,7 @@ private Mono<HttpResponse<String>> sendHttpPost(final String endpoint, final Str
 		return Mono.deferContextual(ctx -> {
 			var builder = this.requestBuilder.copy()
 				.uri(requestUri)
-				.header(HttpHeaders.CONTENT_TYPE, "application/json")
+				.header(HttpHeaders.CONTENT_TYPE, "application/json; charset=utf-8")
 				.header(MCP_PROTOCOL_VERSION_HEADER_NAME, MCP_PROTOCOL_VERSION)
 				.POST(HttpRequest.BodyPublishers.ofString(body));
 			var transportContext = ctx.getOrDefault(McpTransportContext.KEY, McpTransportContext.EMPTY);

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

@@ -99,6 +99,8 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
 
 	private static final String APPLICATION_JSON = "application/json";
 
+	private static final String APPLICATION_JSON_UTF8 = "application/json; charset=utf-8";
+
 	private static final String TEXT_EVENT_STREAM = "text/event-stream";
 
 	public static int NOT_FOUND = 404;
@@ -454,7 +456,7 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
 
 				var builder = requestBuilder.uri(uri)
 					.header(HttpHeaders.ACCEPT, APPLICATION_JSON + ", " + TEXT_EVENT_STREAM)
-					.header(HttpHeaders.CONTENT_TYPE, APPLICATION_JSON)
+					.header(HttpHeaders.CONTENT_TYPE, APPLICATION_JSON_UTF8)
 					.header(HttpHeaders.CACHE_CONTROL, "no-cache")
 					.header(HttpHeaders.PROTOCOL_VERSION,
 							ctx.getOrDefault(McpAsyncClient.NEGOTIATED_PROTOCOL_VERSION,