Remove prefix on over-the-wire ProtocolErrors (#1727)

Co-authored-by: Felix Weinberger <3823880+felixweinberger@users.noreply.github.com>

packages/core/src/types/types.ts

@@ -2340,7 +2340,7 @@ export class ProtocolError extends Error {
         message: string,
         public readonly data?: unknown
     ) {
-        super(`MCP error ${code}: ${message}`);
+        super(message);
         this.name = 'ProtocolError';
     }