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/GatewaySession.cs

@@ -48,6 +48,7 @@ public sealed class GatewaySession
             pipeName,
             nonce,
             clientIdentity,
+            ownerKeyId: null,
             clientSessionName,
             clientCorrelationId,
             commandTimeout,
@@ -66,6 +67,7 @@ public sealed class GatewaySession
     /// <param name="pipeName">Name of the named pipe for gateway-worker IPC.</param>
     /// <param name="nonce">Security nonce for worker validation.</param>
     /// <param name="clientIdentity">Client identity from the authentication context.</param>
+    /// <param name="ownerKeyId">API key identifier of the caller that created this session.</param>
     /// <param name="clientSessionName">Client-supplied session name.</param>
     /// <param name="clientCorrelationId">Client-supplied correlation identifier.</param>
     /// <param name="commandTimeout">Timeout for command invocation.</param>
@@ -79,6 +81,7 @@ public sealed class GatewaySession
         string pipeName,
         string nonce,
         string? clientIdentity,
+        string? ownerKeyId,
         string? clientSessionName,
         string? clientCorrelationId,
         TimeSpan commandTimeout,
@@ -112,6 +115,7 @@ public sealed class GatewaySession
         PipeName = pipeName;
         Nonce = nonce;
         ClientIdentity = clientIdentity;
+        OwnerKeyId = ownerKeyId;
         ClientSessionName = clientSessionName;
         ClientCorrelationId = clientCorrelationId;
         CommandTimeout = commandTimeout;
@@ -148,6 +152,11 @@ public sealed class GatewaySession
     /// </summary>
     public string? ClientIdentity { get; }
 
+    /// <summary>
+    /// Gets the API key identifier of the caller that created this session.
+    /// </summary>
+    public string? OwnerKeyId { get; }
+
     /// <summary>
     /// Gets the client-supplied session name.
     /// </summary>