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/JetStreamStreamLifecycleApiTests.cs

@@ -0,0 +1,18 @@
+using NATS.Server.TestUtilities;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStreamLifecycleApiTests
+{
+    [Fact]
+    public async Task Stream_update_and_delete_roundtrip()
+    {
+        await using var fx = await JetStreamApiFixture.StartWithStreamAsync("ORDERS", "orders.*");
+
+        var update = await fx.RequestLocalAsync("$JS.API.STREAM.UPDATE.ORDERS", "{\"subjects\":[\"orders.v2.*\"]}");
+        update.Error.ShouldBeNull();
+
+        var delete = await fx.RequestLocalAsync("$JS.API.STREAM.DELETE.ORDERS", "{}");
+        delete.Success.ShouldBeTrue();
+    }
+}