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/JetStreamPullConsumerContractTests.cs

@@ -0,0 +1,16 @@
+using NATS.Server.TestUtilities;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamPullConsumerContractTests
+{
+    [Fact]
+    public async Task Pull_fetch_no_wait_returns_immediately_when_empty()
+    {
+        await using var fx = await JetStreamApiFixture.StartWithPullConsumerAsync();
+
+        var batch = await fx.FetchWithNoWaitAsync("ORDERS", "PULL", batch: 1);
+        batch.Messages.Count.ShouldBe(0);
+        batch.TimedOut.ShouldBeFalse();
+    }
+}