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/JetStreamStreamFeatureToggleParityTests.cs

@@ -0,0 +1,30 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Validation;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStreamFeatureToggleParityTests
+{
+    [Fact]
+    public void Stream_feature_toggles_are_preserved_in_config_model_and_validation()
+    {
+        var config = new StreamConfig
+        {
+            Name = "ORDERS",
+            Subjects = ["orders.*"],
+            Sealed = true,
+            DenyDelete = true,
+            DenyPurge = true,
+            AllowDirect = true,
+            MaxMsgSize = 1024,
+            MaxMsgsPer = 10,
+            MaxAgeMs = 5000,
+        };
+
+        JetStreamConfigValidator.Validate(config).IsValid.ShouldBeTrue();
+        config.Sealed.ShouldBeTrue();
+        config.DenyDelete.ShouldBeTrue();
+        config.DenyPurge.ShouldBeTrue();
+        config.AllowDirect.ShouldBeTrue();
+    }
+}