Fix SSE content-type error message format

Author: ddworken

src/server/sse.test.ts

@@ -264,7 +264,7 @@ describe('SSEServerTransport', () => {
         await transport.handlePostMessage(mockReq, mockHandleRes, { jsonrpc: '2.0', method: 'test' });
 
         expect(mockHandleRes.writeHead).toHaveBeenCalledWith(400);
-        expect(mockHandleRes.end).toHaveBeenCalledWith('Error: Content-Type must start with application/json, got: text/plain');
+        expect(mockHandleRes.end).toHaveBeenCalledWith('Error: Unsupported content-type: text/plain');
       });
     });
 

src/server/sse.ts

@@ -155,7 +155,7 @@ export class SSEServerTransport implements Transport {
     try {
       const ct = contentType.parse(req.headers["content-type"] ?? "");
       if (ct.type !== "application/json") {
-        throw new Error(`Unsupported content-type: ${ct}`);
+        throw new Error(`Unsupported content-type: ${ct.type}`);
       }
 
       body = parsedBody ?? await getRawBody(req, {