Fix error swallowing by propagating JSON serialization errors to LLM

server/src/handlers/graph.rs

@@ -380,8 +380,8 @@ impl McpTool for SearchNodesHandler {
                     result.entities.insert(id, e);
                 }
             }
-            serde_json::to_string(&result).unwrap_or_default()
-        });
+            serde_json::to_string(&result).map_err(|e| e.to_string())
+        })?;
         Ok(data)
     }
 }
@@ -418,8 +418,8 @@ impl McpTool for OpenNodesHandler {
                     result.entities.insert(name, e);
                 }
             }
-            serde_json::to_string(&result).unwrap_or_default()
-        });
+            serde_json::to_string(&result).map_err(|e| e.to_string())
+        })?;
         Ok(data)
     }
 }
@@ -620,7 +620,7 @@ impl McpTool for FindOrphansHandler {
                 .collect::<Vec<String>>()
         });
         let data = serde_json::to_string(&orphans).unwrap_or_default();
-        Ok(data.to_string())
+        Ok(data)
     }
 }