List cost report datasets for Cost Explorer

Returns Cost Report read models for the active scope, backing the Cost explorer page on route load. A cost report is a time-bucketed, stacked spend dataset shaped by three query dimensions visible in the UI control bar: groupBy (service|project|region|tag), period (7d|30d|90d|mtd) and granularity (daily|monthly). Each report carries the resolved dimensions (id/name/color), the bucketed series (per-bucket label, ISO date and per-series values in ten-thousandths of the currency unit keyed by series id), the period total and previous-period total (integers in ten-thousandths of the currency unit), and a per-dimension breakdown with current/previous amounts and a sparkline. No resource state is mutated. Validation: scopeId must be accessible (404 NOT_FOUND if unknown); groupBy, period and granularity must be valid enum members when supplied (defaults: service/30d/daily); pagination bounds enforced — violations map to 422 VALIDATION_FAILED. Async & state: Synchronous read; no state change. Datasets reflect the latest completed ingestion window; no SSE frames are tied to this endpoint. Errors: 401 unauthenticated; 403 caller lacks a billing-capable role (owner|admin|contributor|reader|billing) on the scope; 404 unknown scopeId; 422 malformed query parameters; 500 read-model failure. Idempotency/Concurrency: Safe, cacheable; no idempotency key required.