fix: set UTF-8 charset before reading request body in servlet transports

All three servlet-based server transports called request.getReader()
without first setting the character encoding. Per the Jakarta Servlet
spec, getReader() defaults to ISO-8859-1 when the Content-Type header
has no explicit charset parameter. Since application/json without a
charset is valid per RFC 8259, non-ASCII characters in tool names,
argument values, and notification data were silently corrupted.

The analogous server-side fix was applied to StdioServerTransportProvider
in #826 and to the HTTP response path in #881. This commit completes the
fix by applying request.setCharacterEncoding("UTF-8") before getReader()
in HttpServletStreamableServerTransportProvider,
HttpServletSseServerTransportProvider, and
HttpServletStatelessServerTransport.

Fixes #880

Author: Gorre Surya

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

@@ -386,6 +386,7 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 		}
 
 		try {
+			request.setCharacterEncoding(UTF_8);
 			BufferedReader reader = request.getReader();
 			StringBuilder body = new StringBuilder();
 			String line;

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

@@ -154,6 +154,7 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 		}
 
 		try {
+			request.setCharacterEncoding(UTF_8);
 			BufferedReader reader = request.getReader();
 			StringBuilder body = new StringBuilder();
 			String line;

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

@@ -428,6 +428,7 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 		McpTransportContext transportContext = this.contextExtractor.extract(request);
 
 		try {
+			request.setCharacterEncoding(UTF_8);
 			BufferedReader reader = request.getReader();
 			StringBuilder body = new StringBuilder();
 			String line;