feat: update DebugStreamBridgeActor to use gRPC for streaming events

After receiving the initial snapshot via ClusterClient, the bridge actor
now opens a gRPC server-streaming subscription via SiteStreamGrpcClient
for ongoing AttributeValueChanged/AlarmStateChanged events. Adds NodeA/
NodeB failover with max 3 retries, retry count reset on successful event,
and IWithTimers-based reconnect scheduling.

- DebugStreamBridgeActor: gRPC stream after snapshot, reconnect state machine
- DebugStreamService: inject SiteStreamGrpcClientFactory, resolve gRPC addresses
- ServiceCollectionExtensions: register SiteStreamGrpcClientFactory singleton
- SiteStreamGrpcClient: make SubscribeAsync/Unsubscribe virtual for testability
- SiteStreamGrpcClientFactory: make GetOrCreate virtual for testability
- New test suite: DebugStreamBridgeActorTests (8 tests)

src/ScadaLink.Communication/ServiceCollectionExtensions.cs

@@ -1,4 +1,5 @@
 using Microsoft.Extensions.DependencyInjection;
+using ScadaLink.Communication.Grpc;
 
 namespace ScadaLink.Communication;
 
@@ -10,6 +11,7 @@ public static class ServiceCollectionExtensions
             .BindConfiguration("Communication");
 
         services.AddSingleton<CommunicationService>();
+        services.AddSingleton<SiteStreamGrpcClientFactory>();
         services.AddSingleton<DebugStreamService>();
 
         return services;