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/Sessions/ISessionManager.cs

@@ -8,11 +8,13 @@ public interface ISessionManager
     /// <summary>Opens a new gateway session and launches a worker process.</summary>
     /// <param name="request">Request payload.</param>
     /// <param name="clientIdentity">Client identity string.</param>
+    /// <param name="ownerKeyId">API key identifier of the caller creating the session.</param>
     /// <param name="cancellationToken">Token to cancel the asynchronous operation.</param>
     /// <returns>The newly opened session.</returns>
     Task<GatewaySession> OpenSessionAsync(
         SessionOpenRequest request,
         string? clientIdentity,
+        string? ownerKeyId,
         CancellationToken cancellationToken);
 
     /// <summary>Attempts to retrieve a session by ID.</summary>