Count rows of a target entity right now.
the target entity (model's training-unit entity)
OptionalextraFilter: stringoptional filter narrowing the population (mirrors the pipeline scope)
OptionalcontextUser: UserInforequest user — required server-side for isolation/audit
Optionalprovider: IMetadataProvideroptional provider for multi-provider correctness
RunView-backed IRowCounter. Uses thecount_onlyresult type so the full population size lands inTotalRowCountWITHOUT materializing any rows — the efficient way to get a current count for the data-volume / drift triggers.