chore(comm): delete dead IntegrationCallRequest routing (Gitea #32)

'Pattern 4: Integration Routing' — RouteIntegrationCallAsync →
SiteEnvelope(IntegrationCallRequest) → SiteCommunicationActor integration
handler — was plumbed end to end but connected at neither end: no
producer (zero callers) and no handler (AkkaHostedService never
registered LocalHandlerType.Integration). It was an early scaffold the
architecture routed around — the brokered External→Central→Site→Central
round-trip is served by the Inbound API's routed-site-script path (the
RouteTo* verbs, driven from CommunicationServiceInstanceRouter), which is
live, tested, and shares IntegrationTimeout.

Decision (#32): delete. Removed the IntegrationCall{Request,Response}
messages, RouteIntegrationCallAsync, the SiteCommunicationActor receive
block + _integrationHandler field + LocalHandlerType.Integration, and the
four tests that covered them (2 actor, 2 message-contract, 1 dispatcher-
reject, 1 mapper-reject). KEPT IntegrationTimeout — it is the live
timeout for the RouteTo* verbs. Updated the exclusion-narrative comments
(proto/mapper/dispatcher), design §4, the components doc timeout table,
and marked the known-issue RESOLVED.

Full solution build clean (0/0); Communication 634 + Host 421 green.
Net -172/+20 across 14 files. Not in the gRPC proto (was deliberately
excluded there), so no wire-format change.

src/ZB.MOM.WW.ScadaBridge.Communication/Actors/SiteCommunicationActor.cs

@@ -61,12 +61,6 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
     /// <summary>The transport supplied by the Host (null falls back to <see cref="NoOpCentralTransport"/>).</summary>
     private readonly ICentralTransport? _injectedTransport;
 
-    /// <summary>
-    /// Handler for the vestigial <see cref="IntegrationCallRequest"/> — the one command NOT
-    /// migrated to the dispatcher (dead at both ends), so it still routes on the actor.
-    /// </summary>
-    private IActorRef? _integrationHandler;
-
     /// <summary>Akka timer scheduler injected by the framework via <see cref="IWithTimers"/>.</summary>
     public ITimerScheduler Timers { get; set; } = null!;
 
@@ -158,20 +152,6 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
         Receive<RetryParkedOperation>(cmd => DispatchCommand(cmd));
         Receive<DiscardParkedOperation>(cmd => DispatchCommand(cmd));
 
-        // Integration Routing — the 29th command, NOT migrated to the dispatcher (dead at
-        // both ends; no production code registers the handler). Kept on the actor so the
-        // dispatcher's command surface stays the 28 that actually migrate.
-        Receive<IntegrationCallRequest>(msg =>
-        {
-            if (_integrationHandler != null)
-                _integrationHandler.Forward(msg);
-            else
-            {
-                Sender.Tell(new IntegrationCallResponse(
-                    msg.CorrelationId, _siteId, false, null, "Integration handler not available", DateTimeOffset.UtcNow));
-            }
-        });
-
         // Central→site manual failover relay. Central and the site are separate clusters,
         // so central can only ask — this node performs the graceful Leave locally, scoped to
         // the SITE-SPECIFIC role, because that is what site singletons (the Deployment
@@ -281,9 +261,6 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
             case LocalHandlerType.ParkedMessages:
                 _dispatcher.RegisterParkedMessageHandler(msg.Handler);
                 break;
-            case LocalHandlerType.Integration:
-                _integrationHandler = msg.Handler;
-                break;
             case LocalHandlerType.Artifacts:
                 _dispatcher.RegisterArtifactHandler(msg.Handler);
                 break;
@@ -364,6 +341,5 @@ public enum LocalHandlerType
 {
     EventLog,
     ParkedMessages,
-    Integration,
     Artifacts
 }