Implement deferred WaitQueue, DiskAvailability, and NoOpCache behavior with tests

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/DiskAvailabilityTests.cs

@@ -0,0 +1,58 @@
+// Copyright 2012-2026 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+using Shouldly;
+using ZB.MOM.NatsNet.Server;
+
+namespace ZB.MOM.NatsNet.Server.Tests.JetStream;
+
+public sealed class DiskAvailabilityTests
+{
+    private const long JetStreamMaxStoreDefault = 1L * 1024 * 1024 * 1024 * 1024;
+
+    [Fact]
+    public void DiskAvailable_MissingDirectory_ShouldCreateDirectory()
+    {
+        var root = Path.Combine(Path.GetTempPath(), $"disk-avail-{Guid.NewGuid():N}");
+        var target = Path.Combine(root, "nested");
+        try
+        {
+            Directory.Exists(target).ShouldBeFalse();
+
+            var available = DiskAvailability.DiskAvailable(target);
+
+            Directory.Exists(target).ShouldBeTrue();
+            available.ShouldBeGreaterThan(0L);
+        }
+        finally
+        {
+            if (Directory.Exists(root))
+                Directory.Delete(root, recursive: true);
+        }
+    }
+
+    [Fact]
+    public void DiskAvailable_InvalidPath_ShouldReturnFallback()
+    {
+        var available = DiskAvailability.DiskAvailable("\0");
+        available.ShouldBe(JetStreamMaxStoreDefault);
+    }
+
+    [Fact]
+    public void Check_ShouldUseDiskAvailableThreshold()
+    {
+        var root = Path.Combine(Path.GetTempPath(), $"disk-check-{Guid.NewGuid():N}");
+        try
+        {
+            var available = DiskAvailability.DiskAvailable(root);
+
+            DiskAvailability.Check(root, Math.Max(0, available - 1)).ShouldBeTrue();
+            DiskAvailability.Check(root, available + 1).ShouldBeFalse();
+        }
+        finally
+        {
+            if (Directory.Exists(root))
+                Directory.Delete(root, recursive: true);
+        }
+    }
+}