test(sessions): cover OwnerKeyId service-layer forwarding; doc 11-param ctor

Add LastOwnerKeyId capture to FakeSessionManager and assert it equals
"operator01" in OpenSession_WithValidRequest_ReturnsSessionDetails, closing
the gap where OwnerKeyId threading through the service layer had no test
coverage. Add a <remarks> to the 11-param GatewaySession convenience ctor
documenting that OwnerKeyId is null there and authenticated call sites must
use the 12-param overload.

src/ZB.MOM.WW.MxGateway.Server/Sessions/GatewaySession.cs

@@ -30,6 +30,11 @@ public sealed class GatewaySession
     /// <param name="startupTimeout">Timeout for worker process startup.</param>
     /// <param name="shutdownTimeout">Timeout for worker process shutdown.</param>
     /// <param name="openedAt">Timestamp when the session opened.</param>
+    /// <remarks>
+    ///     Constructs a session with no owner key (<see cref="OwnerKeyId"/> will be null).
+    ///     Authenticated call sites that have a resolved API key identity must use the
+    ///     12-parameter overload and pass the caller's key id explicitly.
+    /// </remarks>
     public GatewaySession(
         string sessionId,
         string backendName,