Replace timer-based tests with event-driven patterns

[mcollina]

Summary

This PR eliminates sleep() calls from tests in favor of event-driven patterns, making tests faster and more reliable. Tests now complete exactly when conditions are met instead of waiting for arbitrary timeouts.

Key Changes

New Testing Utilities (test/helpers/events.ts)

Added a comprehensive set of event-driven testing helpers:

• waitForEvents(emitter, event, count): Wait for N events without polling
• createLatch(): Manually resolvable promises for test coordination
• promisifyCallback(subscribe): Convert callback-based APIs to promises
• waitForCallbacks(count): Wait for N callback invocations
• once: Re-exported from Node.js for single-event waiting

Updated Test Files

All test files now use event-driven patterns:

• test/request-response.test.ts: Use waitForEvents() for job completion
• test/deduplication.test.ts: Use waitForEvents() for deduplication checks
• test/reaper.test.ts: Use once() for reaper events
• test/redis-storage.test.ts: Use promisifyCallback() for notifications
• test/file-storage.test.ts: Use once() for file storage events
• test/memory-storage.test.ts: Use once() for memory storage events
• test/integration/e2e.test.ts: Use waitForEvents() for E2E flows

Benefits

• Faster tests: No arbitrary sleep delays (tests complete immediately when conditions are met)
• More reliable: Eliminates race conditions from insufficient sleep delays
• Cleaner resource management: No lingering timers that could prevent test exit
• Better test clarity: Event-driven patterns make test intent explicit

Important Pattern

Event listeners must be registered BEFORE operations that trigger those events to avoid race conditions. For example:

// Correct: Register listener first
const promise = waitForEvents(queue, 'completed', 1)
await queue.enqueue({ type: 'test', data: {} })
await promise

// Incorrect: Race condition risk
await queue.enqueue({ type: 'test', data: {} })
await waitForEvents(queue, 'completed', 1) // Might miss the event!

Exceptions

TTL/expiry tests still use real timers where wall-clock time is required by the feature being tested. Event-driven patterns cannot replace actual time-based expiration logic.

Testing

All existing tests pass with the new event-driven patterns. The test suite should now exit cleanly without any lingering resources.