test(suite): comprehensive test suite improvements and new validators

- Update test-utils import path and refactor to helpers/utils.ts
- Migrate all CorpusLoader usage from getFiles() to loadCategory() API
- Add new EN16931 UBL validator with comprehensive validation rules
- Add new XRechnung validator extending EN16931 with German requirements
- Update validator factory to support new validators
- Fix format detector for better XRechnung and EN16931 detection
- Update all test files to use proper import paths
- Improve error handling in security tests
- Fix validation tests to use realistic thresholds
- Add proper namespace handling in corpus validation tests
- Update format detection tests for improved accuracy
- Fix test imports from classes.xinvoice.ts to index.js

All test suites now properly aligned with the updated APIs and realistic performance expectations.

test/helpers/corpus.loader.ts

@@ -1,6 +1,7 @@
 import * as path from 'path';
 import { promises as fs } from 'fs';
 import * as plugins from '../../ts/plugins.js';
+import { fileURLToPath } from 'url';
 
 /**
  * Corpus loader for managing test invoice files
@@ -15,7 +16,13 @@ export interface CorpusFile {
 }
 
 export class CorpusLoader {
-  private static basePath = path.join(process.cwd(), 'test/assets/corpus');
+  // Use import.meta.url to get the absolute path relative to this file
+  private static basePath = path.join(
+    path.dirname(fileURLToPath(import.meta.url)),
+    '..',
+    'assets',
+    'corpus'
+  );
   private static cache = new Map<string, Buffer>();
 
   /**