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/JetStreamStreamConfigBehaviorTests.cs

@@ -0,0 +1,41 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream;
+using NATS.Server.JetStream.Publish;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStreamConfigBehaviorTests
+{
+    [Fact]
+    public void Stream_honors_dedup_window_and_sealed_delete_purge_guards()
+    {
+        var streamManager = new StreamManager();
+        streamManager.CreateOrUpdate(new StreamConfig
+        {
+            Name = "ORDERS",
+            Subjects = ["orders.*"],
+            DuplicateWindowMs = 10_000,
+            Sealed = false,
+            DenyDelete = false,
+            DenyPurge = false,
+        });
+
+        var publisher = new JetStreamPublisher(streamManager);
+        publisher.TryCaptureWithOptions("orders.created", "one"u8.ToArray(), new PublishOptions { MsgId = "m-1" }, out var first).ShouldBeTrue();
+        publisher.TryCaptureWithOptions("orders.created", "two"u8.ToArray(), new PublishOptions { MsgId = "m-1" }, out var second).ShouldBeTrue();
+        second.Seq.ShouldBe(first.Seq);
+
+        streamManager.CreateOrUpdate(new StreamConfig
+        {
+            Name = "ORDERS",
+            Subjects = ["orders.*"],
+            DuplicateWindowMs = 10_000,
+            Sealed = true,
+            DenyDelete = true,
+            DenyPurge = true,
+        });
+
+        streamManager.DeleteMessage("ORDERS", first.Seq).ShouldBeFalse();
+        streamManager.Purge("ORDERS").ShouldBeFalse();
+    }
+}