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/JetStreamFileStoreRecoveryStrictParityTests.cs

@@ -0,0 +1,37 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Tests.JetStream;
+
+public class JetStreamFileStoreRecoveryStrictParityTests
+{
+    [Fact]
+    public async Task Filestore_recovery_preserves_sequence_subject_index_and_integrity_after_prune_and_restart_cycles()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), $"nats-js-fs-recovery-{Guid.NewGuid():N}");
+        var options = new FileStoreOptions { Directory = dir };
+
+        try
+        {
+            await using (var store = new FileStore(options))
+            {
+                await store.AppendAsync("orders.created", "a"u8.ToArray(), default);
+                await store.AppendAsync("orders.created", "b"u8.ToArray(), default);
+                await store.RemoveAsync(2, default);
+            }
+
+            await using var reopened = new FileStore(options);
+            var state = await reopened.GetStateAsync(default);
+            state.Messages.ShouldBe((ulong)1);
+            state.LastSeq.ShouldBe((ulong)1);
+
+            var msg1 = await reopened.LoadAsync(1, default);
+            msg1.ShouldNotBeNull();
+            msg1.Subject.ShouldBe("orders.created");
+        }
+        finally
+        {
+            if (Directory.Exists(dir))
+                Directory.Delete(dir, recursive: true);
+        }
+    }
+}