Per-site gRPC client for central-side streaming subscriptions to site
servers. SiteStreamGrpcClient manages server-streaming calls with
keepalive, converts proto events to domain types, and supports
cancellation via Unsubscribe. SiteStreamGrpcClientFactory caches one
client per site identifier.
Includes InternalsVisibleTo for test access to conversion helpers and
comprehensive unit tests for event mapping, quality/alarm-state
conversion, unsubscribe behavior, and factory caching.
