Log .ics build failures; add salesforce.client unit tests

Review follow-up:
- tickets.confirmation-email.ts: a failed .ics generation was swallowed
  silently; log a warning (the email still goes out without the
  attachment)
- test/common/salesforce.client.test.ts: direct coverage for the shared
  client's token cache and retry-once-on-401 (previously exercised only
  indirectly through the newsletter sync test)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/tickets/confirmation-email.test.ts

@@ -21,12 +21,14 @@ jest.mock('../../src/middleware/logger', () => ({
 import * as EventsService from '../../src/models/calendar/events/events.service';
 import * as IcalService from '../../src/models/calendar/events/icalgenerator.service';
 import {MailService} from '../../src/common/common.mail';
+import logger from '../../src/middleware/logger';
 import {NachklangTicketsDB} from '../../src/models/tickets/Tickets.db';
 import {recordConfirmationEmailResult, sendRedemptionConfirmation} from '../../src/models/tickets/tickets.confirmation-email';
 
 const mockGetEvent = EventsService.getEventById as jest.Mock;
 const mockToIcal = IcalService.convertToIcal as jest.Mock;
 const mockSendMail = MailService.sendMail as jest.Mock;
+const mockLogger = logger as unknown as {info: jest.Mock; warn: jest.Mock; error: jest.Mock};
 const mockGetConnection = NachklangTicketsDB.getConnection as jest.Mock;
 
 const EVENT = {
@@ -68,13 +70,14 @@ describe('sendRedemptionConfirmation', () => {
 		]);
 	});
 
-	it('still sends (without an attachment) when the .ics build fails', async () => {
+	it('still sends (without an attachment) and warns when the .ics build fails', async () => {
 		mockToIcal.mockRejectedValue(new Error('ical boom'));
 
 		await sendRedemptionConfirmation(RECIPIENT);
 
 		const options = mockSendMail.mock.calls[0][3];
 		expect(options.attachments).toBeUndefined();
+		expect(mockLogger.warn).toHaveBeenCalledWith(expect.stringContaining('without .ics attachment'));
 	});
 
 	it('returns false and does not send when the event no longer exists', async () => {