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/Dashboard/DashboardLiveDataService.cs

@@ -138,6 +138,7 @@ public sealed class DashboardLiveDataService : IDashboardLiveDataService, IAsync
         GatewaySession session = await _sessionManager.OpenSessionAsync(
                 new SessionOpenRequest(BackendName, ClientName, Guid.NewGuid().ToString("N"), CommandTimeout: null),
                 ClientName,
+                ownerKeyId: null,
                 cancellationToken)
             .ConfigureAwait(false);