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/JetStreamConsumerFlowReplayParityTests.cs

@@ -0,0 +1,33 @@
+using NATS.Server.JetStream.Consumers;
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Storage;
+using NATS.Server.JetStream;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamConsumerFlowReplayParityTests
+{
+    [Fact]
+    public void Push_consumer_enqueues_flow_control_and_heartbeat_frames_when_enabled()
+    {
+        var engine = new PushConsumerEngine();
+        var consumer = new ConsumerHandle("ORDERS", new ConsumerConfig
+        {
+            AckPolicy = AckPolicy.Explicit,
+            FlowControl = true,
+            HeartbeatMs = 1000,
+            RateLimitBps = 1024,
+        });
+
+        engine.Enqueue(consumer, new StoredMessage
+        {
+            Sequence = 1,
+            Subject = "orders.created",
+            Payload = "payload"u8.ToArray(),
+        });
+
+        consumer.PushFrames.Count.ShouldBe(3);
+        consumer.PushFrames.Any(f => f.IsFlowControl).ShouldBeTrue();
+        consumer.PushFrames.Any(f => f.IsHeartbeat).ShouldBeTrue();
+    }
+}