fix(mcp): propagate internal serialization errors instead of silently swallowing them to prevent LLM hallucination

nvim-core/src/lib.rs

@@ -684,7 +684,7 @@ async fn execute_nvim_lua_with_args(code: &str, args: Vec<rmpv::Value>) -> Resul
     if let serde_json::Value::String(s) = json_res {
         Ok(s)
     } else {
-        Ok(serde_json::to_string_pretty(&json_res).unwrap_or_default())
+        serde_json::to_string_pretty(&json_res).map_err(|e| e.to_string())
     }
 }