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/FileStoreTests.cs

@@ -0,0 +1,18 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class FileStoreTests
+{
+    [Fact]
+    public async Task FileStore_recovers_messages_after_restart()
+    {
+        var dir = Directory.CreateTempSubdirectory();
+
+        await using (var store = new FileStore(new FileStoreOptions { Directory = dir.FullName }))
+            await store.AppendAsync("foo", "payload"u8.ToArray(), default);
+
+        await using var recovered = new FileStore(new FileStoreOptions { Directory = dir.FullName });
+        (await recovered.GetStateAsync(default)).Messages.ShouldBe((ulong)1);
+    }
+}