fix: validation bugs — If/Switch version check, Set false positive, bare expressions (#675, #676, #677) (#678)

- #675: Wire `validateConditionNodeStructure` into `WorkflowValidator` with
  version-conditional checks (If v2.2+ requires options, v2.0-2.1 validates
  operators, v1.x skipped; Switch v3.2+ requires options)
- #676: Fix `validateSet` to check `assignments.assignments` (v3+) alongside
  `config.values` (v1/v2), eliminating false positive warnings
- #677: Add anchored heuristic pre-pass in `ExpressionValidator` detecting bare
  `$json`, `$node`, `$input`, `$execution`, `$workflow`, `$prevNode`, `$env`,
  `$now`, `$today`, `$itemIndex`, `$runIndex` references missing `={{ }}`

25 new tests across 3 test files.

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

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

tests/unit/services/node-specific-validators.test.ts

@@ -2384,6 +2384,73 @@ return [{"json": {"result": result}}]
     });
   });
 
+  describe('validateSet', () => {
+    it('should not warn when Set v3 has populated assignments', () => {
+      context.config = {
+        mode: 'manual',
+        assignments: {
+          assignments: [
+            { id: '1', name: 'status', value: 'active', type: 'string' }
+          ]
+        }
+      };
+
+      NodeSpecificValidators.validateSet(context);
+
+      const fieldWarnings = context.warnings.filter(w => w.message.includes('no fields configured'));
+      expect(fieldWarnings).toHaveLength(0);
+    });
+
+    it('should not warn when Set v2 has populated values', () => {
+      context.config = {
+        mode: 'manual',
+        values: {
+          string: [{ name: 'field', value: 'val' }]
+        }
+      };
+
+      NodeSpecificValidators.validateSet(context);
+
+      const fieldWarnings = context.warnings.filter(w => w.message.includes('no fields configured'));
+      expect(fieldWarnings).toHaveLength(0);
+    });
+
+    it('should warn when Set v3 has empty assignments array', () => {
+      context.config = {
+        mode: 'manual',
+        assignments: { assignments: [] }
+      };
+
+      NodeSpecificValidators.validateSet(context);
+
+      const fieldWarnings = context.warnings.filter(w => w.message.includes('no fields configured'));
+      expect(fieldWarnings).toHaveLength(1);
+    });
+
+    it('should warn when Set manual mode has no values or assignments', () => {
+      context.config = {
+        mode: 'manual'
+      };
+
+      NodeSpecificValidators.validateSet(context);
+
+      const fieldWarnings = context.warnings.filter(w => w.message.includes('no fields configured'));
+      expect(fieldWarnings).toHaveLength(1);
+    });
+
+    it('should not warn when Set manual mode has jsonOutput', () => {
+      context.config = {
+        mode: 'manual',
+        jsonOutput: '{"key":"value"}'
+      };
+
+      NodeSpecificValidators.validateSet(context);
+
+      const fieldWarnings = context.warnings.filter(w => w.message.includes('no fields configured'));
+      expect(fieldWarnings).toHaveLength(0);
+    });
+  });
+
   describe('validateAIAgent', () => {
     let context: NodeValidationContext;