test: add coverage for Pub/Sub backfill, pre-pull check, and subscriber set logic

Add 14 tests covering backfillPubSubApps (core logic, caching, promise
coalescing), the pre-pull subscription check in PubSubInstance.run(),
and create/update subscriber set conditions. Enhance mock Redis with
key-aware set operations and hmgetBuffer to support the new tests.

Author: andris9