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/JetStreamConsumerDeliverPolicyLongRunTests.cs

@@ -0,0 +1,35 @@
+using NATS.Server.JetStream;
+using NATS.Server.JetStream.Models;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamConsumerDeliverPolicyLongRunTests
+{
+    [Fact]
+    public async Task Deliver_policy_last_per_subject_and_start_time_resolve_consistent_cursor_under_interleaved_subjects()
+    {
+        var streams = new StreamManager();
+        streams.CreateOrUpdate(new StreamConfig
+        {
+            Name = "ORDERS",
+            Subjects = ["orders.*"],
+        }).Error.ShouldBeNull();
+
+        streams.Capture("orders.a", "1"u8.ToArray());
+        streams.Capture("orders.b", "2"u8.ToArray());
+        streams.Capture("orders.a", "3"u8.ToArray());
+
+        var consumers = new ConsumerManager();
+        consumers.CreateOrUpdate("ORDERS", new ConsumerConfig
+        {
+            DurableName = "LAST-B",
+            DeliverPolicy = DeliverPolicy.LastPerSubject,
+            FilterSubject = "orders.b",
+        }).Error.ShouldBeNull();
+
+        var batch = await consumers.FetchAsync("ORDERS", "LAST-B", 1, streams, default);
+        batch.Messages.Count.ShouldBe(1);
+        batch.Messages[0].Subject.ShouldBe("orders.b");
+        batch.Messages[0].Sequence.ShouldBe((ulong)2);
+    }
+}