Implemented cache invalidation for CleanupService to fix stale sandbox paths
being reported after purge() completes. The _sandbox_dirs_cache is now
invalidated after _purge_sandboxes() so subsequent scan() calls re-read the
filesystem instead of returning already-deleted paths.
Changes:
- Added self._sandbox_dirs_cache = None at end of _purge_sandboxes()
- Updated docstring to document cache invalidation behavior
- Created comprehensive BDD test coverage with 5 scenarios under new
features/cleanup_service_cache_invalidation.feature and step definitions
- Updated CHANGELOG.md with bug fix entry
- Updated CONTRIBUTORS.md with PR #8257
ISSUES CLOSED: #7527
