MCP transport error responses expose Java stack traces to the client

The MCP SDK's stateless servlet transport serializes its McpError exception object through the JSON mapper when it rejects a request at the transport level (unsupported Content-Type, missing Accept header, malformed JSON-RPC body, handler failure).

With a default Jackson mapper the response body is a serialized exception of about 20 KB, including the full stack trace with SDK and XWiki class names and line numbers, instead of a JSON-RPC error object. 

Present since the SDK was first integrated, not a 2.1.0 regression.

Upstream PR for this issue already existed.