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/Grpc/SiteStreamGrpcClient.cs

@@ -36,9 +36,10 @@ public class SiteStreamGrpcClient : IAsyncDisposable
     }
 
     /// <summary>
-    /// Private constructor for unit testing without a real gRPC channel.
+    /// Protected constructor for unit testing without a real gRPC channel.
+    /// Allows subclassing for mock implementations.
     /// </summary>
-    private SiteStreamGrpcClient()
+    protected SiteStreamGrpcClient()
     {
     }
 
@@ -62,7 +63,7 @@ public class SiteStreamGrpcClient : IAsyncDisposable
     /// The <paramref name="onEvent"/> callback delivers domain events, and
     /// <paramref name="onError"/> lets the caller handle reconnection.
     /// </summary>
-    public async Task SubscribeAsync(
+    public virtual async Task SubscribeAsync(
         string correlationId,
         string instanceUniqueName,
         Action<object> onEvent,
@@ -109,7 +110,7 @@ public class SiteStreamGrpcClient : IAsyncDisposable
     /// <summary>
     /// Cancels an active subscription by correlation ID.
     /// </summary>
-    public void Unsubscribe(string correlationId)
+    public virtual void Unsubscribe(string correlationId)
     {
         if (_subscriptions.TryRemove(correlationId, out var cts))
         {

src/ScadaLink.Communication/Grpc/SiteStreamGrpcClientFactory.cs

@@ -21,7 +21,7 @@ public class SiteStreamGrpcClientFactory : IAsyncDisposable, IDisposable
     /// <summary>
     /// Returns an existing client for the site or creates a new one.
     /// </summary>
-    public SiteStreamGrpcClient GetOrCreate(string siteIdentifier, string grpcEndpoint)
+    public virtual SiteStreamGrpcClient GetOrCreate(string siteIdentifier, string grpcEndpoint)
     {
         return _clients.GetOrAdd(siteIdentifier, _ =>
         {