style(governance): remove stray whitespace in callback validation

src/va_agent/governance.py

@@ -123,7 +123,7 @@ Devuelve un JSON con la siguiente estructura:
         if callback_context is None:
             error_msg = "callback_context is required"
             raise ValueError(error_msg)
-        
+
         if llm_request is None:
             error_msg = "llm_request is required"
             raise ValueError(error_msg)