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/JetStreamMirrorSourceRuntimeParityTests.cs

@@ -0,0 +1,40 @@
+using NATS.Server.JetStream.MirrorSource;
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamMirrorSourceRuntimeParityTests
+{
+    [Fact]
+    public async Task Mirror_source_runtime_tracks_sync_state_and_subject_mapping()
+    {
+        var mirrorTarget = new MemStore();
+        var sourceTarget = new MemStore();
+        var mirror = new MirrorCoordinator(mirrorTarget);
+        var source = new SourceCoordinator(sourceTarget, new StreamSourceConfig
+        {
+            Name = "SRC",
+            SubjectTransformPrefix = "agg.",
+            SourceAccount = "A",
+        });
+
+        var message = new StoredMessage
+        {
+            Sequence = 10,
+            Subject = "orders.created",
+            Payload = "ok"u8.ToArray(),
+            TimestampUtc = DateTime.UtcNow,
+        };
+
+        await mirror.OnOriginAppendAsync(message, default);
+        await source.OnOriginAppendAsync(message, default);
+
+        mirror.LastOriginSequence.ShouldBe((ulong)10);
+        source.LastOriginSequence.ShouldBe((ulong)10);
+
+        var sourced = await sourceTarget.LoadAsync(1, default);
+        sourced.ShouldNotBeNull();
+        sourced.Subject.ShouldBe("agg.orders.created");
+    }
+}