Resolves StatefulChart/chartAction conflicts by keeping master's async
(202) handling and staleness guards with the legacy explore_json paths
stripped: the endpoint is always /api/v1/chart/data, the async handler
loses its useLegacyApi parameter, and the legacy body-wrapping branch
and its test are removed.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>