fix: convert all test files from jest to vitest and fix QBittorrentClient import

- Convert RTorrentClient.test.js to use vi.mock() instead of jest.mock()
- Convert QBittorrentClient.test.js to use vi.mock() instead of jest.mock()
- Convert SABnzbdClient.test.js to use vi.mock() instead of jest.mock()
- Convert TransmissionClient.test.js to use vi.mock() instead of jest.mock()
- Convert downloadClients.test.js to use vi.mock() instead of jest.mock()
- Convert integration/downloadClients.test.js to use vi.mock() instead of jest.mock()
- Fix legacy qbittorrent.test.js to import QBittorrentClient from new location
- Add getRtorrentInstances mock to downloadClients.test.js
- Add RTORRENT_INSTANCES to integration test environment variables

tests/unit/clients/RTorrentClient.test.js

@@ -1,13 +1,14 @@
 // Copyright (c) 2026 Gordon Bolton. MIT License.
 const RTorrentClient = require('../../../server/clients/RTorrentClient');
 const xmlrpc = require('xmlrpc');
+const { vi } = require('vitest');
 
-jest.mock('xmlrpc', () => ({
-  createClient: jest.fn()
+vi.mock('xmlrpc', () => ({
+  createClient: vi.fn()
 }));
 
-jest.mock('../../../server/utils/logger', () => ({
-  logToFile: jest.fn()
+vi.mock('../../../server/utils/logger', () => ({
+  logToFile: vi.fn()
 }));
 
 describe('RTorrentClient', () => {
@@ -16,7 +17,7 @@ describe('RTorrentClient', () => {
   let mockMethodCall;
 
   beforeEach(() => {
-    mockMethodCall = jest.fn();
+    mockMethodCall = vi.fn();
     xmlrpc.createClient.mockReturnValue({
       methodCall: mockMethodCall
     });
@@ -30,7 +31,7 @@ describe('RTorrentClient', () => {
     };
 
     client = new RTorrentClient(mockConfig);
-    jest.clearAllMocks();
+    vi.clearAllMocks();
   });
 
   describe('Constructor', () => {