feat: implement Phase 5 integration tests (workflow management)

Implemented comprehensive integration tests for workflow deletion and listing:

Test Coverage (16 scenarios):
- delete-workflow.test.ts: 3 tests
  * Successful deletion
  * Error handling for non-existent workflows
  * Cleanup verification

- list-workflows.test.ts: 13 tests
  * No filters (all workflows)
  * Filter by active status (true/false)
  * Filter verification
  * Pagination (first page, cursor, last page)
  * Limit variations (1, 50, 100)
  * Exclude pinned data
  * Empty results
  * Sort order verification

Critical Fixes:
- handleDeleteWorkflow: Now returns deleted workflow data (per n8n API spec)
- handleListWorkflows: Convert tags array to comma-separated string (n8n API format)
- N8nApiClient.deleteWorkflow: Return Workflow object instead of void
- WorkflowListParams.tags: Changed from string[] to string (API expects CSV format)

All 71 integration tests passing.

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

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

src/mcp/handlers-n8n-manager.ts

@@ -607,11 +607,12 @@ export async function handleDeleteWorkflow(args: unknown, context?: InstanceCont
   try {
     const client = ensureApiConfigured(context);
     const { id } = z.object({ id: z.string() }).parse(args);
-    
-    await client.deleteWorkflow(id);
-    
+
+    const deleted = await client.deleteWorkflow(id);
+
     return {
       success: true,
+      data: deleted,
       message: `Workflow ${id} deleted successfully`
     };
   } catch (error) {
@@ -642,12 +643,17 @@ export async function handleListWorkflows(args: unknown, context?: InstanceConte
   try {
     const client = ensureApiConfigured(context);
     const input = listWorkflowsSchema.parse(args || {});
-    
+
+    // Convert tags array to comma-separated string (n8n API format)
+    const tagsParam = input.tags && input.tags.length > 0
+      ? input.tags.join(',')
+      : undefined;
+
     const response = await client.listWorkflows({
       limit: input.limit || 100,
       cursor: input.cursor,
       active: input.active,
-      tags: input.tags,
+      tags: tagsParam as any,  // API expects string, not array
       projectId: input.projectId,
       excludePinnedData: input.excludePinnedData ?? true
     });