fix(historian-gateway): Dispose() delegates to DisposeAsync() + sync-dispose test

Addresses T7/T8/T11 code-review minors: route the sync dispose through DisposeAsync
so a double Dispose()+DisposeAsync() stays a no-op; cover the sync path.

Claude-Session: https://claude.ai/code/session_012SDSQ3AcaXqPcBtDESBRii

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/GatewayHistorianDataSourceTests.cs

@@ -69,6 +69,15 @@ public sealed class GatewayHistorianDataSourceTests
         Assert.Equal(1, fake.DisposeCallCount);
     }
 
+    [Fact]
+    public void Dispose_via_sync_path_disposes_client()
+    {
+        var fake = new FakeHistorianGatewayClient();
+        var ds = new GatewayHistorianDataSource(fake, NullLogger<GatewayHistorianDataSource>.Instance);
+        ((IDisposable)ds).Dispose();
+        Assert.Equal(1, fake.DisposeCallCount);
+    }
+
     // Ts(...) builds a Google.Protobuf.WellKnownTypes.Timestamp from UTC parts.
     private static Timestamp Ts(int y, int mo, int d, int h, int mi, int s)
         => Timestamp.FromDateTime(new DateTime(y, mo, d, h, mi, s, DateTimeKind.Utc));