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/JetStreamIntegrationMatrixTests.cs

@@ -0,0 +1,14 @@
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamIntegrationMatrixTests
+{
+    [Theory]
+    [InlineData("stream-msg-delete-roundtrip")]
+    [InlineData("consumer-msg-next-no-wait")]
+    [InlineData("direct-get-by-sequence")]
+    public async Task Integration_matrix_executes_real_server_scenarios(string scenario)
+    {
+        var result = await JetStreamIntegrationMatrix.RunScenarioAsync(scenario);
+        result.Success.ShouldBeTrue(result.Details);
+    }
+}