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

server/src/handlers/workspaces.rs

@@ -193,7 +193,7 @@ impl McpTool for DeleteSnippetHandler {
             drop(idx.delete_document(&req.name));
             Ok("Snippet deleted.".to_string())
         } else {
-            Ok("Snippet not found.".to_string())
+            Err("Snippet not found.".to_string())
         }
     }
 }