Refactors the notification storage to support multiple stream IDs
using a ConcurrentDictionary. This ensures that the bot can track and
ignore duplicate notifications for multiple streams within a
rolling 24-hour window, rather than only tracking the single most
recent stream.
- Updated ILastPostedStreamStore to include RemoveValuesOlderThan24Hours.
- Switched LastPostedStreamStore to use ConcurrentDictionary with UTC timestamps.
- Injected TimeProvider to enable deterministic testing of time-based expiration.
- Added automatic cache pruning in NotificationHandler on every request.
- Renamed Unit/LastPostedStreamStore.cs to Unit/LastPostedStreamStoreTests.cs and expanded test coverage.
Stevan Freeborn