fix(test): reduce flakiness in TTL boundary test

Change timestamp from `now - 999` to `now - 900` to provide a 100ms
buffer for test execution time. The test was flaky because between
creating the timestamp and checking isFresh(), enough time could pass
to make the record exactly at or past the 1000ms TTL boundary.

This fixes intermittent failures where the test expected true but got
false due to timing issues.

Author: jdalton

packages/cli/test/unit/utils/update/store.test.mts

@@ -255,9 +255,9 @@ describe('store', () => {
 
     it('should handle edge case at TTL boundary', () => {
       const now = Date.now()
-      // Just under 1 second
+      // Well under 1 second with buffer for test execution time.
       const record: StoreRecord = {
-        timestampFetch: now - 999,
+        timestampFetch: now - 900,
         timestampNotification: 0,
         version: '1.0.0',
       }