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/JetStreamMirrorSourceStrictRuntimeTests.cs

@@ -0,0 +1,39 @@
+using NATS.Server.JetStream.MirrorSource;
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Tests.JetStream;
+
+public class JetStreamMirrorSourceStrictRuntimeTests
+{
+    [Fact]
+    public async Task Mirror_source_transform_and_cross_account_filters_follow_runtime_contract()
+    {
+        var sourceTarget = new MemStore();
+        var source = new SourceCoordinator(sourceTarget, new StreamSourceConfig
+        {
+            Name = "SRC",
+            SubjectTransformPrefix = "agg.",
+            SourceAccount = "A",
+        });
+
+        await source.OnOriginAppendAsync(new StoredMessage
+        {
+            Sequence = 1,
+            Subject = "orders.created",
+            Payload = "ok"u8.ToArray(),
+            Account = "A",
+        }, default);
+        await source.OnOriginAppendAsync(new StoredMessage
+        {
+            Sequence = 2,
+            Subject = "orders.created",
+            Payload = "skip"u8.ToArray(),
+            Account = "B",
+        }, default);
+
+        var state = await sourceTarget.GetStateAsync(default);
+        state.Messages.ShouldBe((ulong)1);
+        (await sourceTarget.LoadAsync(1, default))!.Subject.ShouldBe("agg.orders.created");
+    }
+}