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/JetStreamStreamPolicyRuntimeTests.cs

@@ -0,0 +1,24 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.TestUtilities;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStreamPolicyRuntimeTests
+{
+    [Fact]
+    public async Task Discard_new_rejects_publish_when_max_bytes_exceeded()
+    {
+        // Each message = subject("s.a"=3) + payload(2) + overhead(16) = 21 bytes.
+        // MaxBytes=25 holds exactly one message; second publish is rejected.
+        await using var fx = await JetStreamApiFixture.StartWithStreamConfigAsync(new StreamConfig
+        {
+            Name = "S",
+            Subjects = ["s.*"],
+            MaxBytes = 25,
+            Discard = DiscardPolicy.New,
+        });
+
+        (await fx.PublishAndGetAckAsync("s.a", "12")).ErrorCode.ShouldBeNull();
+        (await fx.PublishAndGetAckAsync("s.a", "34")).ErrorCode.ShouldNotBeNull();
+    }
+}