refactor: extract NATS.Server.JetStream.Tests project

Move 225 JetStream-related test files from NATS.Server.Tests into a
dedicated NATS.Server.JetStream.Tests project. This includes root-level
JetStream*.cs files, storage test files (FileStore, MemStore,
StreamStoreContract), and the full JetStream/ subfolder tree (Api,
Cluster, Consumers, MirrorSource, Snapshots, Storage, Streams).

Updated all namespaces, added InternalsVisibleTo, registered in the
solution file, and added the JETSTREAM_INTEGRATION_MATRIX define.

tests/NATS.Server.JetStream.Tests/JetStream/InterestRetentionTests.cs

@@ -0,0 +1,91 @@
+using NATS.Server.JetStream;
+
+namespace NATS.Server.JetStream.Tests.JetStream;
+
+/// <summary>
+/// Tests for InterestRetentionPolicy per-consumer ack tracking.
+/// Go reference: stream.go checkInterestState/noInterest.
+/// </summary>
+public class InterestRetentionTests
+{
+    [Fact]
+    public void ShouldRetain_true_when_consumers_have_not_acked()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.RegisterInterest("consumer-A", "orders.>");
+        policy.RegisterInterest("consumer-B", "orders.>");
+
+        policy.ShouldRetain(1, "orders.new").ShouldBeTrue();
+    }
+
+    [Fact]
+    public void ShouldRetain_false_when_all_consumers_acked()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.RegisterInterest("consumer-A", "orders.>");
+        policy.RegisterInterest("consumer-B", "orders.>");
+
+        policy.AcknowledgeDelivery("consumer-A", 1);
+        policy.ShouldRetain(1, "orders.new").ShouldBeTrue(); // B hasn't acked
+
+        policy.AcknowledgeDelivery("consumer-B", 1);
+        policy.ShouldRetain(1, "orders.new").ShouldBeFalse(); // both acked
+    }
+
+    [Fact]
+    public void ShouldRetain_ignores_consumers_without_interest()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.RegisterInterest("consumer-A", "orders.>");
+        policy.RegisterInterest("consumer-B", "billing.>"); // no interest in orders
+
+        policy.AcknowledgeDelivery("consumer-A", 1);
+        policy.ShouldRetain(1, "orders.new").ShouldBeFalse(); // B has no interest
+    }
+
+    [Fact]
+    public void UnregisterInterest_removes_consumer()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.RegisterInterest("consumer-A", "x.>");
+        policy.RegisterInterest("consumer-B", "x.>");
+
+        policy.UnregisterInterest("consumer-B");
+
+        // Only A needs to ack
+        policy.AcknowledgeDelivery("consumer-A", 1);
+        policy.ShouldRetain(1, "x.y").ShouldBeFalse();
+    }
+
+    [Fact]
+    public void ShouldRetain_false_when_no_consumers_registered()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.ShouldRetain(1, "any.subject").ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Multiple_sequences_tracked_independently()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.RegisterInterest("c1", "x.>");
+
+        policy.AcknowledgeDelivery("c1", 1);
+        policy.ShouldRetain(1, "x.y").ShouldBeFalse();
+        policy.ShouldRetain(2, "x.y").ShouldBeTrue(); // seq 2 not acked
+    }
+
+    [Fact]
+    public void ConsumerCount_tracks_registrations()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.ConsumerCount.ShouldBe(0);
+
+        policy.RegisterInterest("c1", "x.>");
+        policy.RegisterInterest("c2", "y.>");
+        policy.ConsumerCount.ShouldBe(2);
+
+        policy.UnregisterInterest("c1");
+        policy.ConsumerCount.ShouldBe(1);
+    }
+}