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/JetStreamClusterGovernanceRuntimeParityTests.cs

@@ -0,0 +1,26 @@
+using NATS.Server.JetStream.Cluster;
+using NATS.Server.JetStream.Models;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamClusterGovernanceRuntimeParityTests
+{
+    [Fact]
+    public async Task Jetstream_cluster_governance_applies_consensus_backed_placement()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        await meta.ProposeCreateStreamAsync(new StreamConfig
+        {
+            Name = "ORDERS",
+            Subjects = ["orders.*"],
+        }, default);
+
+        var planner = new AssetPlacementPlanner(3);
+        var placement = planner.PlanReplicas(2);
+        var replicas = new StreamReplicaGroup("ORDERS", 1);
+        await replicas.ApplyPlacementAsync(placement, default);
+
+        meta.GetState().Streams.ShouldContain("ORDERS");
+        replicas.Nodes.Count.ShouldBe(2);
+    }
+}