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.Tests/Gateway/Grpc/MxAccessGatewayServiceConstraintTests.cs

@@ -884,10 +884,12 @@ public sealed class MxAccessGatewayServiceConstraintTests
         /// <summary>Opens a test session asynchronously.</summary>
         /// <param name="request">The session open request.</param>
         /// <param name="clientIdentity">The client identity, if any.</param>
+        /// <param name="ownerKeyId">The API key identifier of the caller, if any.</param>
         /// <param name="cancellationToken">Token to observe for cancellation.</param>
         public Task<GatewaySession> OpenSessionAsync(
             SessionOpenRequest request,
             string? clientIdentity,
+            string? ownerKeyId,
             CancellationToken cancellationToken) =>
             Task.FromResult(seededSessions.Values.First());