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/JetStreamConsumerFlowControlParityTests.cs

@@ -0,0 +1,41 @@
+using NATS.Server.JetStream;
+using NATS.Server.JetStream.Models;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamConsumerFlowControlParityTests
+{
+    [Fact]
+    public async Task Push_consumer_emits_flow_control_frames_when_enabled()
+    {
+        var streams = new StreamManager();
+        streams.CreateOrUpdate(new StreamConfig
+        {
+            Name = "ORDERS",
+            Subjects = ["orders.*"],
+        });
+
+        var consumers = new ConsumerManager();
+        consumers.CreateOrUpdate("ORDERS", new ConsumerConfig
+        {
+            DurableName = "PUSH",
+            Push = true,
+            AckPolicy = AckPolicy.Explicit,
+            FlowControl = true,
+            RateLimitBps = 1024,
+        });
+
+        var ack = streams.Capture("orders.created", "x"u8.ToArray());
+        streams.TryGet("ORDERS", out var stream).ShouldBeTrue();
+        var message = await stream.Store.LoadAsync(ack!.Seq, default);
+        message.ShouldNotBeNull();
+        consumers.OnPublished("ORDERS", message!);
+
+        var first = consumers.ReadPushFrame("ORDERS", "PUSH");
+        var second = consumers.ReadPushFrame("ORDERS", "PUSH");
+        first.ShouldNotBeNull();
+        second.ShouldNotBeNull();
+        first!.IsData.ShouldBeTrue();
+        second!.IsFlowControl.ShouldBeTrue();
+    }
+}