feat(grpc): site-side ICentralTransport seam + gRPC transport (T1A.3)

Introduce ICentralTransport as the site->central choke point inside
SiteCommunicationActor. The seven site->central sends (notification submit/
status, audit + cached-telemetry ingest, reconcile, health, heartbeat) now
delegate to an injected transport instead of owning ClusterClient.Send inline.

- AkkaCentralTransport: verbatim extraction of today's ClusterClient.Send path,
  including the exact sender-forwarding that routes central's reply straight
  back to the waiting Ask. Default when no transport is injected -> behaviour
  unchanged, existing SiteCommunicationActorTests pass as-is.
- GrpcCentralTransport + CentralChannelProvider: dial CentralControlService with
  sticky failover + background failback (1s-doubling-cap-60s), PSK +
  x-scadabridge-site via ControlPlaneCredentials, per-call deadlines mirroring
  today's Ask timeouts. Cross-node retry ONLY on provably-unsent
  connect failures; never on DeadlineExceeded. Heartbeat stays fire-and-forget.
- StaticSitePskProvider: site's single own-key provider (fail-closed).
- CommunicationOptions: CentralTransport flag (default Akka) + CentralGrpcEndpoints
  (validator: required when transport=Grpc). Host selects the impl; the
  ClusterClient is created only on the Akka path.

Tests: actor-with-fake-transport (7 delegations + fault routing + heartbeat
no-fault), AkkaCentralTransport sender-forwarding, GrpcCentralTransport over
in-process TestServer (failover flip, sticky, failback, PSK+header, deadline,
no-retry-on-deadline), validator. Communication.Tests 371 green, Host.Tests 391
green; the three above-seam suites pass unmodified.

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/CommunicationOptionsValidatorTests.cs

@@ -104,4 +104,57 @@ public class CommunicationOptionsValidatorTests
         Assert.True(result.Failed);
         Assert.Contains("LiveAlarmCachePublishCoalesce", result.FailureMessage);
     }
+
+    // ── T1A.3: gRPC central transport endpoints (required only when selected) ────
+
+    [Fact]
+    public void GrpcTransport_WithNoEndpoints_IsRejected()
+    {
+        var result = Validate(new CommunicationOptions
+        {
+            CentralTransport = CentralTransportMode.Grpc,
+            CentralGrpcEndpoints = new List<string>(),
+        });
+        Assert.True(result.Failed);
+        Assert.Contains("CentralGrpcEndpoints", result.FailureMessage);
+    }
+
+    [Fact]
+    public void GrpcTransport_WithBlankEndpoint_IsRejected()
+    {
+        var result = Validate(new CommunicationOptions
+        {
+            CentralTransport = CentralTransportMode.Grpc,
+            CentralGrpcEndpoints = new List<string> { "   " },
+        });
+        Assert.True(result.Failed);
+        Assert.Contains("CentralGrpcEndpoints", result.FailureMessage);
+    }
+
+    [Fact]
+    public void GrpcTransport_WithEndpoints_IsValid()
+    {
+        var result = Validate(new CommunicationOptions
+        {
+            CentralTransport = CentralTransportMode.Grpc,
+            CentralGrpcEndpoints = new List<string>
+            {
+                "http://scadabridge-central-a:8083",
+                "http://scadabridge-central-b:8083",
+            },
+        });
+        Assert.True(result.Succeeded, result.FailureMessage);
+    }
+
+    [Fact]
+    public void AkkaTransport_IgnoresMissingGrpcEndpoints()
+    {
+        // The default transport must not be forced to declare gRPC endpoints it never dials.
+        var result = Validate(new CommunicationOptions
+        {
+            CentralTransport = CentralTransportMode.Akka,
+            CentralGrpcEndpoints = new List<string>(),
+        });
+        Assert.True(result.Succeeded, result.FailureMessage);
+    }
 }