Fix TypeScript error in MockSchedulerService test

Use non-null assertion for resolveBlock since TypeScript cannot track
the async reassignment across the setTimeout boundary.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@spotify.com>

packages/backend-test-utils/src/services/MockSchedulerService.test.ts

@@ -261,7 +261,7 @@ describe('MockSchedulerService', () => {
     resolveBlock = undefined;
     const triggerPromise2 = blockingScheduler.triggerTask('blocking');
     await setTimeout(1);
-    resolveBlock?.();
+    resolveBlock!();
     await triggerPromise2;
 
     expect(signals).toHaveLength(3);