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/JetStreamMirrorSourceParityTests.cs

@@ -0,0 +1,38 @@
+using NATS.Server.JetStream;
+using NATS.Server.JetStream.Models;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamMirrorSourceParityTests
+{
+    [Fact]
+    public async Task Source_subject_transform_and_cross_account_mapping_copy_expected_messages_only()
+    {
+        var manager = new StreamManager();
+        manager.CreateOrUpdate(new StreamConfig
+        {
+            Name = "SRC",
+            Subjects = ["orders.*"],
+        });
+
+        manager.CreateOrUpdate(new StreamConfig
+        {
+            Name = "AGG",
+            Subjects = ["agg.*"],
+            Sources =
+            [
+                new StreamSourceConfig
+                {
+                    Name = "SRC",
+                    SubjectTransformPrefix = "agg.",
+                    SourceAccount = "A",
+                },
+            ],
+        });
+
+        manager.Capture("orders.created", "1"u8.ToArray());
+        manager.TryGet("AGG", out var aggregate).ShouldBeTrue();
+        var messages = await aggregate.Store.ListAsync(default);
+        messages.ShouldContain(m => m.Subject == "agg.orders.created");
+    }
+}