feat(sessions): record OwnerKeyId on session creation

Add a nullable string? OwnerKeyId property to GatewaySession that captures
the API key identifier (KeyId) of the authenticated caller that opened the
session. Wire it through ISessionManager.OpenSessionAsync → SessionManager
→ GatewaySession constructor. The gRPC service passes identityAccessor
.Current?.KeyId; internal callers (GatewayAlarmMonitor, DashboardLiveDataService)
pass null. Covers the positive and null cases with two new TDD-first tests.

src/ZB.MOM.WW.MxGateway.Server/Grpc/MxAccessGatewayService.cs

@@ -36,6 +36,7 @@ public sealed class MxAccessGatewayService(
                 .OpenSessionAsync(
                     SessionOpenRequest.FromContract(request),
                     ResolveClientIdentity(),
+                    identityAccessor.Current?.KeyId,
                     context.CancellationToken)
                 .ConfigureAwait(false);