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/StreamStoreContractTests.cs

@@ -0,0 +1,51 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class StreamStoreContractTests
+{
+    [Fact]
+    public async Task Append_increments_sequence_and_updates_state()
+    {
+        var store = new FakeStreamStore();
+        var seq = await store.AppendAsync("foo", "bar"u8.ToArray(), default);
+
+        seq.ShouldBe((ulong)1);
+        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)1);
+    }
+
+    private sealed class FakeStreamStore : IStreamStore
+    {
+        private ulong _last;
+
+        public ValueTask<ulong> AppendAsync(string subject, ReadOnlyMemory<byte> payload, CancellationToken ct)
+        {
+            _last++;
+            return ValueTask.FromResult(_last);
+        }
+
+        public ValueTask<ApiStreamState> GetStateAsync(CancellationToken ct)
+            => ValueTask.FromResult(new ApiStreamState { Messages = _last });
+
+        public ValueTask<StoredMessage?> LoadAsync(ulong sequence, CancellationToken ct)
+            => ValueTask.FromResult<StoredMessage?>(null);
+
+        public ValueTask<StoredMessage?> LoadLastBySubjectAsync(string subject, CancellationToken ct)
+            => ValueTask.FromResult<StoredMessage?>(null);
+
+        public ValueTask<IReadOnlyList<StoredMessage>> ListAsync(CancellationToken ct)
+            => ValueTask.FromResult<IReadOnlyList<StoredMessage>>([]);
+
+        public ValueTask<bool> RemoveAsync(ulong sequence, CancellationToken ct)
+            => ValueTask.FromResult(false);
+
+        public ValueTask PurgeAsync(CancellationToken ct) => ValueTask.CompletedTask;
+
+        public ValueTask<byte[]> CreateSnapshotAsync(CancellationToken ct)
+            => ValueTask.FromResult(Array.Empty<byte>());
+
+        public ValueTask RestoreSnapshotAsync(ReadOnlyMemory<byte> snapshot, CancellationToken ct)
+            => ValueTask.CompletedTask;
+    }
+}