Cache validity now checks _last_coordinator_update (within 30min)
instead of _api_calls_today counter. Fixes false "stale" status
when coordinator runs every 15min but cache validation was only
checking API call counter.
Bug #1: Cache validity shows "stale" at 05:57 AM
Bug #2: Cache age calculation incorrect after midnight turnover
Bug #3: get_cache_validity inconsistent with cache_age sensor
Changes:
- Coordinator: Use _last_coordinator_update for cache validation
- Lifecycle: Extract cache validation to dedicated helper function
- Tests: 7 new tests covering midnight scenarios and edge cases
Impact: Cache validity sensor now accurately reflects coordinator
activity, not just explicit API calls. Correctly handles midnight
turnover without false "stale" status.
