fix(host,deployment-manager,communication): repair cross-module DI regressions from batch 1-2

- DeploymentManager-008: revert IConfiguration overload (violated OptionsTests
  component-convention); Host now binds the ScadaLink:DeploymentManager section
- SiteStreamGrpcServer: make test-only int ctor internal so DI sees one public
  ctor (resolves ambiguous-constructor failure in SiteCompositionRootTests)
- Host site composition-root test config: supply Cluster:SeedNodes for the new
  ClusterOptionsValidator

src/ScadaLink.Communication/Grpc/SiteStreamGrpcServer.cs

@@ -24,7 +24,11 @@ public class SiteStreamGrpcServer : SiteStreamService.SiteStreamServiceBase
     private volatile bool _ready;
     private long _actorCounter;
 
-    public SiteStreamGrpcServer(
+    /// <summary>
+    /// Test-only constructor — kept <c>internal</c> so the DI container sees a
+    /// single public constructor and is not faced with an ambiguous choice.
+    /// </summary>
+    internal SiteStreamGrpcServer(
         ISiteStreamSubscriber streamSubscriber,
         ILogger<SiteStreamGrpcServer> logger,
         int maxConcurrentStreams = 100)