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/JetStreamConfigValidationTests.cs

@@ -0,0 +1,15 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Validation;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamConfigValidationTests
+{
+    [Fact]
+    public void Stream_requires_name_and_subjects()
+    {
+        var config = new StreamConfig { Name = "", Subjects = [] };
+        var result = JetStreamConfigValidator.Validate(config);
+        result.IsValid.ShouldBeFalse();
+    }
+}