fix(mcp): return explicit JSON-RPC errors instead of misleading Ok strings for missing entities and silent graph modification drops

server/src/handlers/meta.rs

@@ -297,7 +297,7 @@ impl McpTool for ResolveTechDebtHandler {
         if found {
             Ok("Tech debt resolved".to_string())
         } else {
-            Ok("Tech debt not found".to_string())
+            Err("Tech debt not found".to_string())
         }
     }
 }