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/JetStreamFileStoreLayoutParityTests.cs

@@ -0,0 +1,33 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamFileStoreLayoutParityTests
+{
+    [Fact]
+    public async Task File_store_uses_block_index_layout_with_ttl_prune_invariants()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), $"natsdotnet-filestore-{Guid.NewGuid():N}");
+        try
+        {
+            await using var store = new FileStore(new FileStoreOptions
+            {
+                Directory = dir,
+                BlockSizeBytes = 128,
+                MaxAgeMs = 60_000,
+            });
+
+            for (var i = 0; i < 100; i++)
+                await store.AppendAsync($"orders.{i}", "x"u8.ToArray(), default);
+
+            var state = await store.GetStateAsync(default);
+            state.Messages.ShouldBe((ulong)100);
+            store.BlockCount.ShouldBeGreaterThan(1);
+        }
+        finally
+        {
+            if (Directory.Exists(dir))
+                Directory.Delete(dir, recursive: true);
+        }
+    }
+}