Implement in-process multi-dataset sync isolation across core, network, persistence, and tests

tests/ZB.MOM.WW.CBDDC.Core.Tests/DatasetAwareModelTests.cs

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+using System.Text.Json;
+using ZB.MOM.WW.CBDDC.Core.Storage;
+
+namespace ZB.MOM.WW.CBDDC.Core.Tests;
+
+public class DatasetAwareModelTests
+{
+    [Fact]
+    public void DocumentMetadata_ShouldDefaultDatasetId_ToPrimary()
+    {
+        var metadata = new DocumentMetadata("Users", "42", new HlcTimestamp(100, 0, "node"));
+
+        metadata.DatasetId.ShouldBe(DatasetId.Primary);
+    }
+
+    [Fact]
+    public void DocumentMetadata_SerializationRoundTrip_ShouldPreserveDatasetId()
+    {
+        var original = new DocumentMetadata("Users", "42", new HlcTimestamp(100, 0, "node"), false, "logs");
+
+        string json = JsonSerializer.Serialize(original);
+        var restored = JsonSerializer.Deserialize<DocumentMetadata>(json);
+
+        restored.ShouldNotBeNull();
+        restored.DatasetId.ShouldBe("logs");
+    }
+
+    [Fact]
+    public void SnapshotMetadata_ShouldDefaultDatasetId_ToPrimary()
+    {
+        var metadata = new SnapshotMetadata();
+
+        metadata.DatasetId.ShouldBe(DatasetId.Primary);
+    }
+
+    [Fact]
+    public void PeerOplogConfirmation_ShouldDefaultDatasetId_ToPrimary()
+    {
+        var confirmation = new PeerOplogConfirmation();
+
+        confirmation.DatasetId.ShouldBe(DatasetId.Primary);
+    }
+}