Add unit tests for delivery pipeline

[franrob-projects]

The delivery pipeline (src/lib/slackSender.ts, src/lib/teamsSender.ts, src/lib/delivery.ts) has no tests. Add tests covering:

• buildDeliveryContent() returns correct structure for different module IDs
• sendSlackReminder() handles missing token gracefully
• sendTeamsReminder() handles missing webhook URL gracefully
• Delivery log is written on success and failure

Use Vitest or Jest. Mock the Slack WebClient and fetch for Teams.