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/JetStreamStreamApiTests.cs

@@ -0,0 +1,17 @@
+using NATS.Server.TestUtilities;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStreamApiTests
+{
+    [Fact]
+    public async Task Stream_create_and_info_roundtrip()
+    {
+        var create = await JetStreamApiFixture.RequestAsync("$JS.API.STREAM.CREATE.ORDERS", "{\"name\":\"ORDERS\",\"subjects\":[\"orders.*\"]}");
+        create.Error.ShouldBeNull();
+
+        var info = await JetStreamApiFixture.RequestAsync("$JS.API.STREAM.INFO.ORDERS", "{}");
+        info.Error.ShouldBeNull();
+        info.StreamInfo!.Config.Name.ShouldBe("ORDERS");
+    }
+}