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/MemStoreTests.cs

@@ -0,0 +1,20 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class MemStoreTests
+{
+    [Fact]
+    public async Task MemStore_supports_append_load_and_purge()
+    {
+        var store = new MemStore();
+        var seq1 = await store.AppendAsync("a", "one"u8.ToArray(), default);
+        var seq2 = await store.AppendAsync("a", "two"u8.ToArray(), default);
+
+        seq2.ShouldBe(seq1 + 1);
+        (await store.LoadAsync(seq2, default))!.Payload.Span.SequenceEqual("two"u8).ShouldBeTrue();
+
+        await store.PurgeAsync(default);
+        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)0);
+    }
+}