fix: convert test files to ES modules and fix qbittorrent test method calls

- Convert all client test files from CommonJS require() to ES module import syntax
- Convert downloadClients.test.js and integration/downloadClients.test.js to ES modules
- Fix qbittorrent.test.js to use getActiveDownloads() instead of getTorrents()
- All test files now use proper Vitest-compatible ES module syntax
- Resolves Vitest import errors and QBittorrentClient method call errors

tests/unit/clients/RTorrentClient.test.js

@@ -1,7 +1,7 @@
 // Copyright (c) 2026 Gordon Bolton. MIT License.
-const RTorrentClient = require('../../../server/clients/RTorrentClient');
-const xmlrpc = require('xmlrpc');
-const { vi } = require('vitest');
+import RTorrentClient from '../../../server/clients/RTorrentClient.js';
+import xmlrpc from 'xmlrpc';
+import { vi } from 'vitest';
 
 vi.mock('xmlrpc', () => ({
   createClient: vi.fn()