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/JetStreamStreamRuntimeParityTests.cs

@@ -0,0 +1,29 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Publish;
+using NATS.Server.JetStream.Validation;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamStreamRuntimeParityTests
+{
+    [Fact]
+    public void Stream_runtime_enforces_retention_and_size_preconditions()
+    {
+        var invalid = new StreamConfig
+        {
+            Name = "ORDERS",
+            Subjects = ["orders.*"],
+            Retention = RetentionPolicy.WorkQueue,
+            MaxConsumers = 0,
+            MaxMsgSize = -1,
+        };
+
+        var result = JetStreamConfigValidator.Validate(invalid);
+        result.IsValid.ShouldBeFalse();
+
+        var preconditions = new PublishPreconditions();
+        preconditions.Record("m1", 1);
+        preconditions.IsDuplicate("m1", duplicateWindowMs: 10_000, out var existing).ShouldBeTrue();
+        existing.ShouldBe((ulong)1);
+    }
+}