feat(grpc): add RunWriteOnSession seam for write-reuse spike

src/AVEVA.Historian.Client/Grpc/HistorianGrpcHistoricalWriteOrchestrator.cs

@@ -47,6 +47,24 @@ internal sealed class HistorianGrpcHistoricalWriteOrchestrator
         HistorianGrpcHandshake.Session session = HistorianGrpcHandshake.OpenSession(
             connection, _options, cancellationToken,
             connectionMode: HistorianWcfAuthChainHelper.NativeIntegratedWriteEnabledConnectionMode);
+        return RunWriteOnSession(connection, session, tag, values, cancellationToken);
+    }
+
+    // Spike/Phase-1 seam (A1): run the historical write against an EXTERNALLY-supplied, already-
+    // authenticated write-enabled (0x401) connection + session — NO Create()/handshake here. Run()
+    // delegates so the per-call path and the reuse path share one write implementation (DRY).
+    internal bool RunWriteOnSession(
+        HistorianGrpcConnection connection,
+        HistorianGrpcHandshake.Session session,
+        string tag,
+        IReadOnlyList<HistorianHistoricalValue> values,
+        CancellationToken cancellationToken)
+    {
+        if (values.Count == 0)
+        {
+            return true;
+        }
+
         string handle = session.StringHandle;
         DateTime Deadline() => DateTime.UtcNow.Add(_options.RequestTimeout);