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/JetStreamStartupTests.cs

@@ -0,0 +1,14 @@
+using NATS.Server.TestUtilities;
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStartupTests
+{
+    [Fact]
+    public async Task JetStream_enabled_server_starts_service()
+    {
+        await using var server = await TestServerFactory.CreateJetStreamEnabledAsync();
+        await server.WaitForReadyAsync();
+
+        server.Stats.JetStreamEnabled.ShouldBeTrue();
+    }
+}