fix: raise session timeout default, fix VS Code MCP compatibility (#674)

* fix: raise session timeout default and fix VS Code MCP compatibility (#626, #600, #611)

- #626: Raise SESSION_TIMEOUT_MINUTES default from 5 to 30 minutes.
  Complex editing sessions easily exceed 5 min between LLM calls.

- #600: Add z.preprocess(tryParseJson, ...) to operations parameter
  in n8n_update_partial_workflow. VS Code extension sends arrays as
  JSON strings.

- #611: Strip undefined values from tool args via JSON round-trip
  before Zod validation. VS Code sends explicit undefined which
  Zod's .optional() rejects.

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

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

* refactor: deduplicate tryParseJson — export from handlers-n8n-manager

tryParseJson was duplicated in handlers-workflow-diff.ts. Now imported
from handlers-n8n-manager.ts where it was already defined. Updated
test mock to use importOriginal so the real function is available.

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

---------

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

tests/unit/mcp/handlers-workflow-diff.test.ts

@@ -17,9 +17,13 @@ vi.mock('@/services/workflow-diff-engine');
 vi.mock('@/services/n8n-api-client');
 vi.mock('@/config/n8n-api');
 vi.mock('@/utils/logger');
-vi.mock('@/mcp/handlers-n8n-manager', () => ({
-  getN8nApiClient: vi.fn(),
-}));
+vi.mock('@/mcp/handlers-n8n-manager', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('@/mcp/handlers-n8n-manager')>();
+  return {
+    ...actual,
+    getN8nApiClient: vi.fn(),
+  };
+});
 
 // Import mocked modules
 import { getN8nApiClient } from '@/mcp/handlers-n8n-manager';