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/JetStreamApiRouterCoverageTests.cs

@@ -0,0 +1,32 @@
+using NATS.Server.JetStream;
+using NATS.Server.JetStream.Api;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamApiRouterCoverageTests
+{
+    [Theory]
+    [InlineData("$JS.API.STREAM.UPDATE.ORDERS")]
+    [InlineData("$JS.API.STREAM.DELETE.ORDERS")]
+    [InlineData("$JS.API.STREAM.PURGE.ORDERS")]
+    [InlineData("$JS.API.CONSUMER.DELETE.ORDERS.DUR")]
+    [InlineData("$JS.API.CONSUMER.MSG.NEXT.ORDERS.DUR")]
+    public void Router_recognizes_remaining_subject_families(string subject)
+    {
+        var streams = new StreamManager();
+        _ = streams.CreateOrUpdate(new NATS.Server.JetStream.Models.StreamConfig
+        {
+            Name = "ORDERS",
+            Subjects = ["orders.*"],
+        });
+        var consumers = new ConsumerManager();
+        _ = consumers.CreateOrUpdate("ORDERS", new NATS.Server.JetStream.Models.ConsumerConfig
+        {
+            DurableName = "DUR",
+        });
+
+        var router = new JetStreamApiRouter(streams, consumers);
+        var response = router.Route(subject, "{}"u8);
+        response.Error.ShouldBeNull();
+    }
+}