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/SessionManager.cs

@@ -58,11 +58,13 @@ public sealed class SessionManager : ISessionManager
     /// </summary>
     /// <param name="request">Session open request.</param>
     /// <param name="clientIdentity">Client authentication identity.</param>
+    /// <param name="ownerKeyId">API key identifier of the caller creating the session.</param>
     /// <param name="cancellationToken">Cancellation token.</param>
     /// <returns>Opened gateway session.</returns>
     public async Task<GatewaySession> OpenSessionAsync(
         SessionOpenRequest request,
         string? clientIdentity,
+        string? ownerKeyId,
         CancellationToken cancellationToken)
     {
         ArgumentNullException.ThrowIfNull(request);
@@ -72,7 +74,7 @@ public sealed class SessionManager : ISessionManager
         bool sessionOpenedRecorded = false;
         try
         {
-            session = CreateSession(request, clientIdentity);
+            session = CreateSession(request, clientIdentity, ownerKeyId);
             if (!_registry.TryAdd(session))
             {
                 throw new SessionManagerException(
@@ -420,7 +422,8 @@ public sealed class SessionManager : ISessionManager
 
     private GatewaySession CreateSession(
         SessionOpenRequest request,
-        string? clientIdentity)
+        string? clientIdentity,
+        string? ownerKeyId)
     {
         string sessionId = CreateSessionId();
         string backendName = string.IsNullOrWhiteSpace(request.RequestedBackend)
@@ -441,6 +444,7 @@ public sealed class SessionManager : ISessionManager
             pipeName,
             nonce,
             clientIdentity,
+            ownerKeyId,
             request.ClientSessionName,
             clientCorrelationId,
             commandTimeout,