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/JetStreamStorageSelectionTests.cs

@@ -0,0 +1,13 @@
+using NATS.Server.TestUtilities;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStorageSelectionTests
+{
+    [Fact]
+    public async Task Stream_with_storage_file_uses_filestore_backend()
+    {
+        await using var fx = await JetStreamApiFixture.StartWithStreamJsonAsync("{\"name\":\"S\",\"subjects\":[\"s.*\"],\"storage\":\"file\"}");
+        (await fx.GetStreamBackendTypeAsync("S")).ShouldBe("file");
+    }
+}