chart_data_export now registers lifecycle callback for immediate
updates when coordinator data changes ("fresh" lifecycle state).
Previously only updated via polling intervals.
Changes:
- Register callback in sensor constructor (when entity_key matches)
- Callback triggers async_write_ha_state() on "fresh" lifecycle
- 5 new tests covering callback registration and triggering
Impact: Chart data export updates immediately on API data arrival,
enabling real-time dashboard updates without polling delay.
