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/JetStreamFileStoreCryptoCompressionTests.cs

@@ -0,0 +1,33 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamFileStoreCryptoCompressionTests
+{
+    [Fact]
+    public async Task File_store_compression_and_encryption_roundtrip_preserves_payload()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), $"natsdotnet-filestore-crypto-{Guid.NewGuid():N}");
+        try
+        {
+            await using var store = new FileStore(new FileStoreOptions
+            {
+                Directory = dir,
+                EnableCompression = true,
+                EnableEncryption = true,
+                EncryptionKey = [1, 2, 3, 4],
+            });
+
+            var payload = Enumerable.Repeat((byte)'a', 512).ToArray();
+            var seq = await store.AppendAsync("orders.created", payload, default);
+            var loaded = await store.LoadAsync(seq, default);
+            loaded.ShouldNotBeNull();
+            loaded.Payload.ToArray().ShouldBe(payload);
+        }
+        finally
+        {
+            if (Directory.Exists(dir))
+                Directory.Delete(dir, recursive: true);
+        }
+    }
+}