test: update server error test to expect actual error message

The test was expecting the old generic 'Please try again later or contact support'
message, but we now return the actual error message from the N8nServerError
('Internal server error') for better debugging.

This aligns with our change to make error messages more helpful by showing
the actual server error instead of a generic message.

tests/unit/mcp/handlers-workflow-diff.test.ts

@@ -499,7 +499,7 @@ describe('handlers-workflow-diff', () => {
 
       expect(result).toEqual({
         success: false,
-        error: 'n8n server error. Please try again later or contact support.',
+        error: 'Internal server error',
         code: 'SERVER_ERROR',
       });
     });