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/JetStreamStreamPolicyParityTests.cs

@@ -0,0 +1,38 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStreamPolicyParityTests
+{
+    [Fact]
+    public async Task Stream_rejects_oversize_message_and_prunes_by_max_age_and_per_subject_limits()
+    {
+        var streamManager = new StreamManager();
+        var create = streamManager.CreateOrUpdate(new StreamConfig
+        {
+            Name = "P",
+            Subjects = ["p.*"],
+            MaxMsgSize = 8,
+            MaxAgeMs = 20,
+            MaxMsgsPer = 1,
+        });
+        create.Error.ShouldBeNull();
+
+        var oversized = streamManager.Capture("p.a", "0123456789"u8.ToArray());
+        oversized.ShouldNotBeNull();
+        oversized!.ErrorCode.ShouldBe(10054);
+
+        streamManager.Capture("p.a", "one"u8.ToArray())!.ErrorCode.ShouldBeNull();
+        streamManager.Capture("p.a", "two"u8.ToArray())!.ErrorCode.ShouldBeNull();
+
+        streamManager.TryGet("P", out var handle).ShouldBeTrue();
+        var beforeAgePrune = await handle.Store.GetStateAsync(default);
+        beforeAgePrune.Messages.ShouldBe((ulong)1);
+
+        await Task.Delay(30);
+        streamManager.Capture("p.b", "x"u8.ToArray())!.ErrorCode.ShouldBeNull();
+        var afterAgePrune = await handle.Store.GetStateAsync(default);
+        afterAgePrune.Messages.ShouldBe((ulong)1);
+    }
+}