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/test.conversion.ts

@@ -1,7 +1,7 @@
 import { tap, expect } from '@git.zone/tstest/tapbundle';
 import { EInvoice, EInvoiceFormatError } from '../ts/index.js';
 import { InvoiceFormat } from '../ts/interfaces/common.js';
-import { TestFileHelpers, TestFileCategories, PerformanceUtils, TestInvoiceFactory } from './test-utils.js';
+import { TestFileHelpers, TestFileCategories, PerformanceUtils, TestInvoiceFactory } from './helpers/utils.js';
 import * as path from 'path';
 
 /**
@@ -320,7 +320,7 @@ tap.test('Conversion - Error handling and recovery', async () => {
   
   try {
     await invalidInvoice.exportXml('facturx');
-    expect.fail('Should have thrown an error for invalid invoice');
+    throw new Error('Should have thrown an error for invalid invoice');
   } catch (error) {
     console.log(`✓ Invalid invoice error caught: ${error.message}`);