v2.6.1-beta.8

Fix: add unit_class to cost statistic metadata

• Added unit_class=monetary to cost statistic metadata in get_cost_statistic_metadata()
• Fixes HA deprecation warning: doesn't specify unit_class when calling async_add_external_statistics (will stop working in HA 2026.11)

Full Changelog: v2.6.1-beta.7...v2.6.1-beta.8