query_range does not forward unsupported queries #304

--forward-unsupported-queries works for /api/v1/query but not /api/v1/query_range. When a range query can't be served from sketches, the engine returns an error body instead of proxying to Prometheus. Since Grafana dashboards are almost entirely query_range, this breaks transparent fallback for all unaccelerated panels.

Relevant code: process_range_query_request in asap-query-engine/src/drivers/query/servers/http.rs — there's even a comment: "For now, return error - fallback for range queries can be added later."