fix: filter out description field in workflow updates (issue #431)

Root Cause:
- n8n recently added a `description` field to workflows
- The field is returned by GET /workflows/:id endpoint
- But PUT /workflows/:id rejects it with "additional properties" error
- Our cleanWorkflowForUpdate() wasn't filtering it out

The Fix:
1. Added `description` to the list of removed fields in cleanWorkflowForUpdate()
2. Added `description` field to Workflow type definition
3. Added test case to verify description field is excluded

Code Review Improvements:
- Removed diagnostic logging code (was only needed for debugging)
- Improved comment accuracy in type definition
- Added comprehensive test coverage for description field

Testing:
- Build succeeds
- New test passes: "should exclude description field for n8n API compatibility"
- All unit tests pass
- Integration tests confirmed fix works with real n8n API

Related: #431

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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

src/services/n8n-validation.ts

@@ -140,6 +140,7 @@ export function cleanWorkflowForUpdate(workflow: Workflow): Partial<Workflow> {
     // Remove fields that cause API errors
     pinData,
     tags,
+    description, // Issue #431: n8n returns this field but rejects it in updates
     // Remove additional fields that n8n API doesn't accept
     isArchived,
     usedCredentials,

src/types/n8n-api.ts

@@ -56,6 +56,7 @@ export interface WorkflowSettings {
 export interface Workflow {
   id?: string;
   name: string;
+  description?: string; // Returned by GET but must be excluded from PUT/PATCH (n8n API limitation, Issue #431)
   nodes: WorkflowNode[];
   connections: WorkflowConnection;
   active?: boolean; // Optional for creation as it's read-only

tests/unit/services/n8n-validation.test.ts

@@ -367,6 +367,22 @@ describe('n8n-validation', () => {
         expect(cleaned.name).toBe('Test Workflow');
       });
 
+      it('should exclude description field for n8n API compatibility (Issue #431)', () => {
+        const workflow = {
+          name: 'Test Workflow',
+          description: 'This is a test workflow description',
+          nodes: [],
+          connections: {},
+          versionId: 'v123',
+        } as any;
+
+        const cleaned = cleanWorkflowForUpdate(workflow);
+
+        expect(cleaned).not.toHaveProperty('description');
+        expect(cleaned).not.toHaveProperty('versionId');
+        expect(cleaned.name).toBe('Test Workflow');
+      });
+
       it('should omit settings property when no settings provided (Issue #431)', () => {
         const workflow = {
           name: 'Test Workflow',