fix: code validator false positives and null property removal (#294, #293, #611) (#637)

- Fix $() node reference triggering "Invalid $ usage" warning by adding ( and _ to regex lookahead
- Fix helper function primitive returns triggering "Cannot return primitive values" error
- Fix null values in diff engine causing Zod validation errors — null now deletes properties
- Update property removal docs from undefined to null

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dist/services/workflow-diff-engine.d.ts

@@ -3,6 +3,10 @@ import { Workflow } from '../types/n8n-api';
 export declare class WorkflowDiffEngine {
     private renameMap;
     private warnings;
+    private modifiedNodeIds;
+    private removedNodeNames;
+    private tagsToAdd;
+    private tagsToRemove;
     applyDiff(workflow: Workflow, request: WorkflowDiffRequest): Promise<WorkflowDiffResult>;
     private validateOperation;
     private applyOperation;