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/JetStreamStreamRuntimePolicyLongRunTests.cs

@@ -0,0 +1,12 @@
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStreamRuntimePolicyLongRunTests
+{
+    [Fact]
+    public async Task Stream_runtime_policy_guards_hold_under_repeated_publish_cycles()
+    {
+        var baseline = new JetStreamStreamPolicyParityTests();
+        for (var i = 0; i < 3; i++)
+            await baseline.Stream_rejects_oversize_message_and_prunes_by_max_age_and_per_subject_limits();
+    }
+}